Even for a man who has broken all the rules of American politics, the gamble that Donald Trump is taking this summer and fall, beginning this week in Ohio, is astonishing.

The U.S. President waded into the last special election of the season during a rally on Saturday night, saying he needed more Republicans in Congress – a precursor to a full-throttle effort for the fall midterm elections that Mr. Trump says will have him on the campaign trail ‘’six or seven days a week.’’

The danger for the President in an Ohio riding that the Republican congressional candidate carried by a landslide 67 per cent in 2016, and that Mr. Trump himself carried by 11 percentage points, is that a defeat there would embolden Democrats who are hoping to build a ‘’blue wave’’ to take over the House of Representatives – a development that could lead to Mr. Trump’s impeachment. And with a poll last week showing the race, between Republican Troy Balderson and Democrat Danny O’Connor, a virtual tie, Mr. Trump’s involvement ahead of the Tuesday vote has both special potential and special peril.

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The district includes the state capital of Columbus and the Ohio State University, and the area’s voters are unusually politically sophisticated and wealthier than those in Ohio’s 15 other ridings. Neither the names of Mr. Trump, reviled by Democrats, nor former House Speaker Nancy Pelosi, a special target of opprobrium among Republicans, are on the ballot – but their names are on the lips of the candidates, giving this contest the air of a referendum on national politics, and on Mr. Trump in particular.

Other presidents who transformed American politics – Woodrow Wilson, Franklin Roosevelt and Ronald Reagan – have tried what Mr. Trump is doing, running a highly visible, and highly risky, campaign to elect political allies to the Congress. Each of his predecessors failed miserably – and suffered substantial consequences.

Mr. Wilson’s Democrats lost control of the Senate, dooming his dream to win American admission to the League of Nations that he largely designed himself. FDR’s Democrats also were repudiated, losing 71 House seats, ruining his plan to purge his party of conservatives, and making it impossible for him to extend his New Deal agenda. Mr. Reagan’s Republicans lost 26 seats, erasing the gains the GOP had made on the strength of the 40th president’s victory over Jimmy Carter two years earlier and stymying efforts to make the Republicans the natural party of government in the 1980s.

There is one important peculiarity in this contest in central Ohio: Mr. Balderson has the strong support both of Mr. Trump and of Governor John Kasich. Mr. Kasich, who ran against Mr. Trump in 2016, remains a bitter critic of the President, and is contemplating a challenge to the President in 2020. The involvement of Mr. Kasich, who for 18 years held the House seat Mr. Balderson seeks, is a reflection of internal Ohio politics: In a television advertisement that began airing last week, Mr. Kasich praised the congressional candidate as ‘’a partner of mine’’ in reducing state taxes ‘’and turning Ohio around.’’

For his part, Mr. O’Connor is a traditional moderate Democrat, without the more liberal views on health care and free university education that helped Alexandria Ocasio-Cortez win an important New York primary earlier this summer and catapult her to national prominence. He is engaged to a Republican woman and is aiming his appeal to the political independents who may hold the balance of power in Tuesday’s balloting.

By jumping into the race in Ohio – and by plunging into the contest on the last weekend of campaigning – Mr. Trump is taking special risks.

If Mr. Balderson wins, Democrats and commentators will very likely dismiss his victory as the logical result in a riding that is unequivocally Republican. But if Mr. Balderson is defeated, Mr. Trump and the Republicans will be on the defensive, having lost a seat that Republican Representative Pat Tiberi, who resigned to become president of the Ohio Business Roundtable, held for 16 years.

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The link between Mr. Balderson and Mr. Trump was on full display Saturday night, when the President hailed him as ‘’really tough’’ and ‘’really smart.’’ The link between Mr. O’Connor and Ms. Pelosi, one of the principal themes of Republican advertisements, is tenuous, for although the President claimed Mr. O’Connor would be “a puppet of Nancy Pelosi,” the Democratic candidate has pledged not to support the veteran California lawmaker for speaker if the Democrats retake the House in November.

This special election is a curtain-raiser to far more important contests in the fall for two reasons. The first is that it is but the appetizer to the political entree in November, when all 435 House seats and a third of the Senate are up for grabs in the broader midterm congressional elections. The second is a special feature of this contest: Regardless of who wins, the two men will compete for the seat again three months from now.