It will be Hillary Clinton and Donald Trump battling each other in a bruising general election campaign to become the next U.S. president.
Ms. Clinton reached the delegates needed to become the presumptive nominee on Monday night, according to a tally by Associated Press. Her rival, Mr. Trump, crossed the delegate threshold on May 26.
The moment is steeped in symbolism: Ms. Clinton will be the first woman to lead a major U.S. political party in a presidential campaign.
And it comes as the marathon U.S. primaries draw to an end – a four-month-long roller-coaster ride that showcased a daily soap opera of Donald Trump upending the Republican Party establishment and Hillary Clinton trying to fend off an insurgent socialist rival who captivated millennials.
There are half a dozen states where voters go to the polls today, the last big day of multi-state voting.
But the stage is largely set: once Ms. Clinton and Mr. Trump are officially chosen by their respective party conventions in July, they will face off against each other in a campaign that will test the boundaries of civility and force voters to choose between two controversial candidates.
As the primaries end and make way for the July party conventions and the hotly contested general election in the autumn, The Globe and Mail asked four political experts to share the key takeaways and lessons they gleaned from watching each party, the candidates and the nominating process – and what it could mean for the Clinton-Trump match-up.
Key takeaways: The Republican Party
1. The unravelling of the Grand Old Party
The decades-old Republican coalition that brought together libertarian voters in the west and once-Democratic white southern voters is coming undone, according to Lilly Goren, a professor of political science at Carroll University in Waukesha, Wisc.
What the party eventually becomes is anyone's guess. Republican orthodoxies of small government and free trade going back to the Ronald Reagan era are being tested by Mr. Trump's anti-globalization and big government rhetoric.
Many in the libertarian wing of the party object to the idea of building a wall along the U.S.-Mexican border and sending undocumented residents to countries like Mexico, Prof. Goren said.
"If you're going to get rid of 11 million undocumented individuals, you have to increase the size of the federal government substantially, and it's going to cost a lot of money. For the deficit hawks out there, that's a huge price tag," she said.
Mr. Trump's big-government ideas and anti-trade positions could cost him in the general election among some Republican voters, as well as donors such as the billionaire Koch brothers. "The Koch brothers are big free marketeers and they see a lot of what Trump is talking about, getting into trade wars with China, as really problematic if you're a free marketeer," Prof. Goren said.
Mr. Trump's anti-trade themes will also put Ms. Clinton is an awkward spot. The Democratic presumptive nominee belongs to the wing of the party that is largely pro-free trade, although she came out against the Trans-Pacific Partnership trade agreement last October at a time when the party's base expressed strong misgivings.
2. The polls told everyone so
Fuelled by skepticism over a candidate who seemed more entertainer than politician and whose controversial comments about Mexicans and Muslims would have disqualified any other candidate from the race, few political experts and scholars paid attention to the poll numbers.
"He's been leading [in the polls] from the beginning and I think for many people because it felt so implausible that Donald Trump, the guy from The Apprentice and not-so-reputable businessman billionaire, that he could actually be leading the Republican Party primary contests and because of the things he said, people dismissed it," said Leah Wright Rigueur, an assistant professor of public policy at the Harvard Kennedy School.
Polls should be treated with caution. For example, some of the Trump-versus-Clinton polling match-ups show Mr. Trump leading by wide a margin. But these polls were being conducted in the middle of a primary process in which Ms. Clinton was still battling to become the Democratic nominee.
Once she is the presumptive nominee, the data will be more reliable, Prof. Rigueur said. Look to the end of July and early August – after the party conventions – for those polls.
3. How Donald Trump tapped into 'nativism'
The fact that many in the Republican primary electorate were deeply anti-establishment came as no surprise. Public approval of U.S. Congress barely breaks double digits at 11 per cent, according to Gallup.
What many political experts and observers underestimated is the undertones of racism and nativism that motivated segments of the electorate – in particular, many Trump voters, according to Kelly Dittmar, an assistant professor of political science at Rutgers University.
Mr. Trump tapped into those currents – likening undocumented Mexicans to rapists and zeroing in on Muslims. "Who would have thought that saying banning all Muslims would actually help you in a campaign?" Prof. Dittmar said.
But the exits polls showed that Republican primary voters generally backed Mr. Trump's positions.
Prof. Dittmar draws a distinction between American nationalism and nativism.
"With nationalism, generally you'd say it is pride in country. But nativism is a pride in country of who belongs and who doesn't. And in the context of this election, it's about white Americans feeling like it's our country and there are others," she said.
"On a basic level, creating that dichotomy between yourself and the other … that you are the pure American and that there are other people infringing on that, whether that be economically, whether that be through immigration, and that it creates separate Americas," she added.
4. How to stop the next Donald Trump
Ever since the modern primary system came in to existence in 1972, party elites and the public have come to the same decision about who should be the party's presidential nominee, said Elaine Kamarck, a senior fellow at the Brookings Institution.
But 2016 is a different story.
"The Republicans got taken over. Their public process worked in a way that gave them somebody that none of the elites in the party would have chosen," Dr. Kamarck said.
In other words, the voters went one way and the party establishment resisted and tried to go in a different direction – and eventually gave up trying.
That parting of ways between the Republican primary electorate and the elites is going to have to be addressed ahead of the 2020 primaries, Dr. Kamarck argued.
"Parties tend to react to what happens in the general election. If Donald Trump loses and takes down with him Republican control of the senate – suppose Republicans lose 10 to 20 seats in the House – there will be a discussion in the Republican Party about whether or not they should do something to keep another Trump from being nominated," she said.
"If he wins the presidency, that discussion won't happen," she added.
One idea, according to Dr. Kamarck, would be to introduce superdelegates to the Republican nominating contests – ensuring that party officials hold the balance of decision-making power if another Trump-like candidate emerges.
Key takeaways: The Democratic Party
1. The fight against income inequality
The story of the nominating contests is how the fight against income inequality is now centre stage in Democratic policy debates.
"The contest between Hillary Clinton and Bernie Sanders, but particularly the popularity of Bernie Sanders's message, has been about drawing out the fact that there are a lot of people in the United States who are concerned about income inequality and the need for the political system to respond to that," Carroll University's Prof. Goren said.
Both candidates are calling for raising the minimum wage to $15 in order to help working Americans.
Discussions about the gap between the rich and the poor are nothing new in Democratic circles. What makes 2016 unique is how income inequality and the minimum wage have become galvanizing issues, Prof. Goren said.
2. A stronger Democratic presidential nominee
There has been a lot of hand-wringing over the drawn-out Democratic primary contest because it delays Ms. Clinton's pivot to the general election and the real fight ahead against Mr. Trump.
Even as Ms. Clinton is poised to make history on Tuesday night, easily crossing the delegate threshold to become the party's presumptive nominee and using the occasion to mark the historic moment with a speech that will be widely watched, Bernie Sanders refuses to go quietly. He has promised to continue all the way to the convention.
"For him, this is about forcing through the policy issues that he cares about on to the program committee and on to the Democratic platform. So he's pushing Hillary Clinton to the last and forcing her to be a stronger candidate," Harvard University's Prof. Rigueur said.
Mr. Sanders is also trying to gain a foothold at the Democratic Party convention through the diverse coalition of loyal millennials that has propelled his insurgent candidacy. The Clinton campaign will need the energy and enthusiastic backing of this group in the general election fight.
"They're saying if you want our vote, Hillary Clinton, after this point, you need to work for it and we need to see a platform that's inclusive," Prof. Rigueur said.
Ms. Clinton will need to work hard to mobilize and win-over millennials, a key subset of the Obama coalition that helped propel the party to victories in 2008 and 2012. They will be critical once again in the general election campaign of 2016.
3. A long way from post-gender politics
Double standards abound when it comes to women and men in politics, and 2016 shows that there is much work to do, according to Rutgers University's Prof. Dittmar.
Take the yelling Hillary Clinton trope: Mr. Trump and a cast of political commentators have regularly pointed out that the Democratic candidate's campaign speeches can sound like shouting.
"Is she yelling sometimes? Definitely. But why are we so much more offended – and why do we talk so much about it – when it's a female candidate versus a male candidate?" Prof. Dittmar said.
Ms. Clinton will face significant hurdles as she steps into general election mode against Mr. Trump. "The presidency is such a masculanized space. The credentials that we look for in a president have been for so long associated with men and masculinity," Prof. Dittmar said.
For Ms. Clinton, the challenge is clear: "Prove that you're man enough be president and that you're woman enough as a woman candidate," she said.
Mr. Trump – known to give his rivals belittling nicknames – has dubbed Ms. Clinton "crooked Hillary."
The aura of lying, dishonesty, secrecy and scandal has plagued the Clintons for decades, Prof. Dittmar pointed out.
But the "crooked Hillary" label gets at a larger point: Research shows that women are expected to be more honest and ethical than men, and when they fall short, the consequences can be more severe, she said.
4. How the Democratic Party blocked an outsider
In a year that saw outsider candidates try to grab control of the Republican and Democratic nominating contests, the Democratic Party can argue that it successfully fended off an insurgent campaign to upend the party favourite.
Much of that comes down to how the Democratic Party assigns its delegates.
Candidates are awarded delegates based on the share of the vote they win in a primary contest. There are no winner-take-all contests that could really ignite a campaign.
One of the critiques of the Democratic process is the outsized role of superdelegates – the party officials and elites – that overwhelming backed Ms. Clinton in 2016 and helped her become the presumptive nominee.
A reduced role for superdelegates may have worked in Mr. Sanders's advantage, and could help a future outsider. But it is not likely.
"No one is very interested in throwing their U.S. senator or governor or congressman out of the convention. And the people who make these decisions are members of the DNC [Democratic National Committee] and they are not interested in throwing themselves out of the convention," the Brookings Institution's Dr. Kamarck said.