In doing so, they’re under pressure to stick to the facts while explaining their ambitious agendas and defending their records despite a debate format that lends itself to short answers and sound bites rather than nuanced policy discussions.

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“One way to appeal to voters who are wavering about the way Trump operates is to play the Jimmy Carter card and make this like Watergate,” said Barbara Perry, a presidential historian at the University of Virginia’s Miller Center of Public Affairs. “It’s ‘if you’re really sick of the Richard Nixon’s lies, then vote for me, Jimmy Carter, and I will never lie to you.’ ”

In the weeks leading up to the debates, candidates have been blocking off chunks of time to huddle with staff, read briefing books and hone their answers on subjects likely to come up during the events. During two two-hour sessions Wednesday and Thursday, Democratic contenders will have to balance criticizing Trump, while also accurately explaining their own proposals on health care, taxes, immigration, climate change and more.

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They’ll have to do it with a mix of 60-seconds answers and 30-seconds rebuttals.

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Several candidates have sought to cast themselves as honest brokers capable of standing up to a president who has been willing to challenge basic facts and realities.

Harris regularly introduces issues on the campaign trail by saying “Let’s speak truth,” a mantra that has become the centerpiece of her stump speech.

“With this president, we’re in a fight for truth itself,” Harris said on Twitter earlier this month. “As our nation’s leaders continue to lie and undermine our democratic institutions, I know that we are better than this.”

The Washington Post’s Fact Checker’s database has documented more than 10,000 false or misleading claims by Trump since he took office, and the pace of falsehoods has been increasing in recent months.

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Some of these statements have taken on outsize importance after the investigation by special counsel Robert S. Mueller III revealed a pattern of dishonesty that Democrats and hundreds of former prosecutors say served to advance the president’s attempt to obstruct justice.

Several Democratic presidential contenders have said Congress should begin impeachment hearings against Trump for obstruction of justice charges, a move that House Speaker Nancy Pelosi (D-Calif.) has so far resisted.

A Trump campaign official declined to comment.

Democrats can expect to be challenged on their adherence to the facts as well.

The Republican National Committee will have a war room set up during both nights of the debate, offering live fact checks and commentary to counterprogram Democratic messaging. A party official said the RNC will highlight any inaccuracies, misleading statements or reversals by candidates.

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Some Democrats have strayed from the truth in their attempts to attack Trump’s record, or to defend their own. They’ve regularly sought to downplay positive economic news during the Trump administration, countering the president’s hyperbole with false statements of their own.

Speaking about Trump’s tax cuts in April, former vice president Joe Biden falsely claimed that “all of it went to folks at the top and corporations.” While businesses and wealthy individuals benefited disproportionately from the 2017 tax bill, other Americans also received a tax cut.

On the campaign trail, Harris and Sanders have lamented the fact that millions of people are working more than one job, despite low unemployment in an economy Trump has hailed. But only about 5 percent of people work more than one job, and the number of Americans working two full-time jobs has been declining in recent years. Harris appeared to briefly remove the reference from her stump speech a day after The Post published a fact check. The California senator also conceded an error after The Post’s Fact Checker questioned her claim that she had not supported a 2015 bill in California to ensure independent investigations when police use fatal force because she never took positions on bills as attorney general. In fact, Harris regularly issued news releases announcing her support for various bills.

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While Democrats are unified in criticizing the president, the debates will likely expose some intraparty fissures and force candidates to defend their policies and their records.

Liberal candidates who support transformational policies like Medicare-for-all will be pushed to explain how they would structure and pay for them. Moderate candidates will have to defend their records and explain past votes that are now out of step with the party.

Former Maryland congressman John Delaney, who will be on the debate stage Wednesday, plans to call out his fellow Democrats over their health-care plans, said his campaign spokesman, Michael Hopkins.

“We need a president who is going to tell the truth,” he said. “We fully intend on making clear that people who support Medicare-for-all support a program that will get rid of private insurance. And anyone who says otherwise is being disingenuous.”

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Several candidates, including Sen. Elizabeth Warren of Massachusetts and Cory Booker of New Jersey, have co-sponsored Sanders’s Medicare-for-all legislation, but haven’t fully embraced its principles on the campaign trail. Sanders has said that under his bill, which creates a “single-payer” government-run health-care system, private insurance would be essentially eliminated.

Democrats have shied away from publicly supporting the prospect of eliminating a private insurance system that currently covers most Americans. But the debates could force the issue, as moderate candidates plan to press their rivals to take a stand.

Straying from the truth during a debate can prove politically damaging for candidates, some of whom will be introducing themselves to a national audience for the first time.

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During a presidential debate in 2012, Republican candidate Mitt Romney struggled after a CNN moderator corrected him on his claim that President Barack Obama hadn’t referred to a 2012 consulate attack in Benghazi, Libya, as an “act of terror.”

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Sen. Ted Cruz (R-Tex.) had to defend himself from charges of dishonesty during the 2016 Republican presidential primary. Then-candidate Trump nicknamed him “Lyin’ Ted.”

Cruz found himself on the defensive during a January 2016 debate when Sen. Marco Rubio (R-Fla.) criticized his record on defense spending and immigration.

“At least half of the things Marco said are flat-out false,” Cruz said during the debate. “They’re absolutely false.”

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Republican candidates seeking to call out Trump in 2016 were not able to effectively make a case to voters on the issue of honesty. Trump beat Cruz, Rubio and more than a dozen other candidates en route to the nomination. In the general election, he defeated Hillary Clinton, who also tried to make honesty a campaign issue but suffered from attacks on her trustworthiness.

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Democrats running in 2020 have said now that Trump has a record as president, he is vulnerable to criticism that he has repeatedly misled the American people.

Mueller’s investigation revealed episodes of Trump directing his aides to give false information to journalists and the public about his efforts to fire the special counsel and other matters. The president also dictated a misleading statement about a 2016 meeting between campaign officials and Russians in Trump Tower, the report found.

“Whether you’re conservative or moderate or progressive, I don’t think the American people are proud that we have a president who was a pathological liar,” Sanders said during a town hall on Fox News in April.

For his part, Trump has said he will live-tweet during the Democrats’ debates.

He has already tried to brand Democrats as dishonest, seizing on Warren’s claim of Indian heritage, Biden’s record on criminal justice and trade, and other candidates’ support for free college and health care.

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Perry, the presidential historian, said Trump’s willingness to attack his opponents over honesty despite his own record of falsehoods highlights how difficult it will be for Democrats to break through the president’s flood-the-zone strategy.

“It puts anyone who’s trying to deal with that back on their heels, because you’re trying to do it in real time,” she said. “The best politician for the Trump era would be one that has no personal skeletons, no political scandals, and can be as straightforward as possible and as accurate as possible.”