The rollercoaster race is about to get crazier. What's next?

So you thought the last Trump-Clinton presidential debate was jaw-dropping?

What comes next may well dwarf the nastiness showcased before tens of millions of TV viewers during the second presidential debate.

The battle for the White House will be fought on the airwaves, on the ground and once again on the debate stage. Here are the key moments to watch for in the closing weeks of the U.S. presidential campaign.

1. The final debate: Trump Warfare, Part Two

Hillary Clinton weathered an onslaught of venom-laced Trumpian charges in the second presidential debate.

She was accused of being a liar, having hate in her heart, someone who tore down the women that accused her husband of sexual misconduct, and destined for jail over her handling of a controversial private e-mail server during her time time as U.S. secretary of state.

That is quite a list, and frankly Donald Trump could add to it in the final presidential debate on Oct. 19 at the University of Nevada in Las Vegas.

Call it "Trump Warfare": The Republican presidential candidate is conducting a nothing-to-lose campaign that paints Ms. Clinton as untrustworthy, incompetent and unworthy of the Oval Office.

The final presidential debate will energize his base of supporters and motivate them to get out to vote. But it will not sway new voters his way.

An estimated 66.5 million people watched the second presidential debate, according to Nielsen data released Monday. That is down from the record 84 million people that watched the first debate on Sept. 26. The first debates are generally more widely viewed. But the drop is significant.

It may be that viewers got what they needed from the first debate. Or, perhaps more likely, they are increasingly turned off by the tenor of the presidential campaign and the choice they must make.

2. The early-voting drive

The Clinton campaign is also about getting the base out. These Democratic voters came out in droves in the last two presidential elections for President Barack Obama.

For Ms. Clinton, it is all about mobilizing the Obama coalition: African-American, Latino and young college-educated white women voters.

The Clinton campaign is executing a robust on-the-ground operation to get voters registered, to get them to vote during early voting days, and then get the stragglers out to the polling booth on the actual voting day on Nov. 8.

Ms. Clinton appeared at a massive Monday rally with nearly 20,000 people in Ohio the day after the second presidential debate for a reason: The voter registration deadline is today .

It also why she is campaigning in Florida today with former vice-president Al Gore. The voter registration deadline there is on Wednesday.

And it continues: Ms. Clinton is swinging through Colorado and Nevada this week. Both are battleground states where the Clinton campaign is hoping to safeguard its voter advantage by getting voters out early rather than waiting until voting day.

The message at these rallies follows a script. Check the Democratic Party-run website to make sure voters are registered and persuade families and friends to sign up before the deadline.

Early voting accounted for 35 per cent of the national votes four years ago with 34 states allowing advance voting. In 2016, it could be as high as 40 per cent with 37 states allowing voters to mail-in or cast their ballot in person during early-voting days.

The conventional wisdom among pundits is that Ms. Clinton will clinch the White House through early voting.

3. The avalanche of polling

National polls show Ms. Clinton leading Mr. Trump by 11 percentage points in a four-way race that includes the Libertarian and Green presidential candidates, according to NBC/Wall Street Journal poll that surveyed likely voters on Saturday and Sunday. In other words, after the Trump video controversy but before the second presidential debate.

Not all polls show that kind of Clinton lead. The Politico/Morning Consult poll released Tuesday shows a Clinton five-point advantage in a four-way race.

But polling in battleground states shows a tighter picture, even if Ms. Clinton holds the advantage:

• In Ohio, Ms. Clinton is leading 46-42 in a two-way race among likely voters, according to a CBS/YouGov poll released Sunday.
• In Pennsylvania, Ms. Clinton is leading 48-40 in a two-way race among likely voters, according to the same poll.
• In Florida, Ms. Clinton is leading 46-44 in a two-way race among likely voters, according to an NBC/Wall Street Journal poll released Sunday.

Keep an eye out for battleground-state polls. Ultimately, it is a handful of states that will decide the election. Where the race stands in those states will determine how resources are deployed by each campaign.

That includes high-profile surrogates like the Obamas and Vice-President Joe Biden. Michelle Obama is campaigning in battleground New Hampshire on Thursday to remind voters that in that state there is same-day voter registration.

She will also likely weigh in on the Trump video controversy in a highly anticipated moment.

Also watch for the battleground states where the campaigns buy TV ad time.

That will offer a clue about where they need to shore up support and their strategy. The latest bunch of Clinton campaign ads, for example, take aim at moderate Republicans.

The calculation is that these lifelong Republicans are horrified by Mr. Trump and need to be eased into the Clinton column.

4. The Republican exodus

Speaker of the U.S. House of Representatives Paul Ryan says he will no longer defend the Republican presidential candidate or appear with him on the campaign trail.

Just think about that.

One of the most senior figures of the Republican Party is virtually walking away from Mr. Trump and telling his caucus that they must do what they think is right for themselves and winning their congressional district.

THE REPUBLICAN CRISIS

Read Joanna Slater's story: GOP in turmoil as Trump threatens to bring party to its knees.

The worry here is that the Trump train wreck will demolish the Republican Party's chances of holding both chambers of Congress and serving as a check on a Clinton presidency.

Mr. Trump was on Twitter Tuesday morning lashing out at Mr. Ryan, and celebrating what he sees as his new-found freedom.

At least 20 Republican figures un-endorsed Mr. Trump after the 2005 video of his lewd remarks about women became public.

The last two Republican presidential nominees, Mitt Romney and John McCain, have ditched Mr. Trump. The question now is: Who is left?

There are two living U.S. Republican presidents that have largely stayed silent. If George H.W. Bush and his son come out strongly against Mr. Trump and advise voters to back Ms. Clinton, that would be unprecedented, although not that far-fetched.

It will not change the minds of Trump supporters. But it could be the push moderate lifelong Republicans need to vote Democratic, at least when it comes to the presidential race.

With a report from Associated Press

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