Democratic candidates on the sidelines of the Trump-Biden Ukraine showdown are confronting an unmistakable tension in their campaigns: How far to go in their defense of a presidential rival.

Most of Biden’s chief opponents are renewing their calls for Trump’s impeachment after the president allegedly pressured Ukrainian leaders to open an investigation into Biden and his son in exchange for hundreds of millions of dollars in U.S. military aid. They’ve blasted the president for “violating our constitutional mandates” and denounced his appeals for the probe as a “political threat.”

But they’re facing growing calls from diverse voices in the party to do even more, including rallying around Biden in more forceful ways for the good of the country — and at possible expense to their own campaigns. Compounding the pressure is Biden’s position atop the field, and their need for him to fall in order for them to gain.

Aides to 10 Democratic candidates acknowledged the internal strain in interviews with POLITICO, stressing they need to avert a repeat of 2016, when Trump capitalized on Hillary Clinton’s use of a private email server. Several expressed sensitivities around the candidates saying anything that could be construed as helping advance Trump and his allies’ storylines. Yet the candidates and their aides have mostly tried to turn the conversation back toward Trump’s actions, rather than speaking up for Biden.

“I think they should go out of their way to defend Biden,” James Carville, the former senior strategist to Bill Clinton, said of Biden's rivals. Carville has joined party elders to implore House Democrats to formally launch impeachment proceedings. “That is the smart political play because he’s being attacked by Trump. If you are being attacked by a guy that is 100% unfavorable it should not hurt you.”

“We know what happened, OK?” Carville added, citing the lack of evidence that Biden did anything improper. “There’s nothing to be cute about. This is an outrage.”

Still, some campaigns pushed back on the idea of giving blanket cover to Biden when the issue remains a moving target — and while there may be non-Ukraine-related activity that could plunge him into trouble and hamstring his bid. They believe Trump and his personal attorney, Rudy Giuliani, have acted so outrageously that even potential credible, non-biased reporting on the subject in the future might be easy for Biden to brush aside.

Bernie Sanders’ remarks on the subject seemed to underscore the pressure Biden’s opponents are under. At a campaign stop Tuesday in Iowa, Sanders reiterated his calls for an impeachment inquiry and said he believes the president has committed impeachable offenses. But Sanders said he wants to see more evidence before opining on whether Trump should be impeached.

“Trump likely engaged in obstruction of justice with regard to the Mueller investigation,” Sanders said, adding, “We appear to have a president who unbelievably may have used security funds ... as a means to gain political dirt on an opponent.”

Sanders was asked by a reporter whether he considers Hunter Biden fair game for scrutiny, particularly after Sanders urged Americans to move on from the Clinton email story in 2016. Sanders said he prefers to see evidence before weighing in.

“You make the judgment,” Sanders balked in response to a separate question about whether it makes Biden a weaker candidate. “You guys write very perceptive articles on these things.”

The Trump-Ukraine story has dominated the headlines for days and pushed candidates into sometimes uncomfortable territory. As the story broke last week, Kamala Harris accused the president of attacking American democracy and urged he “leave Joe alone,” one of the sturdiest defenses to date from one of his Democratic opponents.

On Monday in Los Angeles, Harris called the possibility of an investigation into Joe or Hunter Biden part of a “political threat," saying, “I have no support for it.”

Yet asked whether the Ukraine story adds to the “narrative” that Biden has baggage, Harris deflected: “I’ll leave that to the voters to decide.”

Part of the tension stems from the reality that much of the field needs Biden’s campaign to collapse. But it would be poor form — and could come off as un-American — for a rival campaign to look like they’re capitalizing on Trump’s gambit. Warnings have come from some unlikely quarters: The Intercept, a liberal publication that's been critical of Biden’s campaign, carried a column by the writer Robert Mackey under the headline: “Reporters should stop helping Donald Trump spread lies about Joe Biden and Ukraine.”

“Any candidate that were to echo Trump’s bad-faith attack would be ending their campaign at that very moment,” said Dan Pfeiffer, a former Obama adviser and co-host of “Pod Save America.” “[Julián] Castro's approval numbers in the Iowa Poll should be a bright flashing red light for anyone that attacks a fellow Democrat in a ‘Trumpian’ fashion.”

Pfeiffer advised the Democratic candidates to defend Biden “and then move on to the fact that the president has admitted to committing multiple crimes beyond the pale of anything Nixon did in Watergate.”

Pete Buttigieg, aboard his media bus tour of Iowa, nodded in that direction, arguing that Democrats shouldn’t allow Trump to change the subject to Hunter Biden after the president himself acted nefariously.

“I haven’t seen any indication of anything there, anyway,” Buttigieg said of Hunter, before pivoting to how he thinks Democrats should deal with Trump’s tactics.

“We’ve got to remember that they’ll either find a vulnerability or they’ll invent one for everybody,” Buttigieg said. “I hope this doesn’t get in our heads more than the most glaring issue that deserves attention, which is the fact that the president of the United States has confessed to a nefarious act. … It’s a big deal. And I don’t think we should talk past that question until we get the answer.”

Democrats moved in that direction Tuesday, as House Speaker Nancy Pelosi’s long-running efforts to resist an impeachment inquiry came to an end. Biden himself said Congress has no choice but to impeach Trump if he doesn’t comply with ongoing investigations, including into Ukraine.

Several of the 2020 Democratic campaign aides said they’d continue to put the onus on Trump and impeachment, a big focus of late for Harris, as well as Elizabeth Warren and Cory Booker. Late Monday, the New Jersey senator accused Trump, who authorized the release of a transcript of his call with Ukrainian President Volodymyr Zelensky, of “acting more like a dictator.”

Without mentioning Biden, Booker called it a “moral moment” to hold the president accountable.

Holly Otterbein and Elena Schneider contributed to this report.