(Bloomberg) -- Joe Biden has so far survived attacks over his son’s work for a Ukrainian company that were designed to weaken his presidential prospects, but that resilience could soon face its toughest test.

Republicans are threatening to subpoena Biden’s 49-year-old son Hunter as a witness in the impeachment trial of President Donald Trump if Democrats succeed in getting testimony from former White House National Security Advisor John Bolton. That would force a spotlight on to an issue that the Biden campaign has worked furiously to knock down -- just days before the first contest to choose a nominee, the Feb. 3 Iowa caucuses.

Nationally, Biden’s front-runner status has held steady, with support ranging roughly between 26% and 30% nationally in most polls since September, when the publication of a whistle-blower complaint brought renewed national attention to Hunter Biden’s work on the board of Ukrainian gas company Burisma Holdings while his father was in office.

Since then, the president’s claim that Biden behaved unethically to benefit his son has been roundly debunked, though it’s continued to circulate among Trump’s allies. Trump lawyer Pam Bondi made Hunter Biden’s position on Burisma’s board while his father took the lead on U.S. policy toward Ukraine the focal point of her presentation to the Senate.

Democratic strategists haunted by damaging right-wing attacks on past nominees Hillary Clinton and John Kerry say the specter of Hunter Biden getting publicly grilled by Senate Republicans would inject uncertainty into a still-fluid primary race.

“It’s the same playbook as ‘Clinton Cash,’ the same playbook as the email and Benghazi attacks – to raise as many doubts as possible, among as many people as possible, that Joe Biden is corrupt, which is laughable,” says Robert Shrum, chief strategist for Kerry’s 2004 presidential campaign and director of the Center for the Political Future at the University of Southern California. He was referring to the best-selling 2015 book by conservative author Peter Schweizer that tarnished Clinton’s image.

“I don’t think it will work, because Democrats are inoculated to Trump,” he said. “But I can’t know that for sure. Any testimony would get wall to wall coverage, no question. And it could bleed off into independent voters.”

Shrum said the right-wing “Swift Boat Veterans for Truth” attack on Kerry’s war record in 2004 disrupted his presidential campaign just as it was gaining steam. The attack initially wasn’t covered by mainstream media, except for Fox News. Then, as it picked up energy, the networks covered it. In a close race Kerry lost, he said, that may have been decisive.

A decision on whether to call witnesses won’t be made before Friday. Democrats are pressing to bring in Bolton after the New York Times reported that his forthcoming book says Trump directly linked the release of aid for Ukraine to investigations of the Bidens. At least three GOP senators have said they’re interested in hearing from Bolton and several others say they are undecided.

But Trump’s allies vow that if Bolton is called, they’ll subpoena Hunter Biden, and perhaps the former vice president.

“If you call John Bolton, we’re going to call everybody,” South Carolina Senator Lindsey Graham, one of Trump’s most vocal defenders, said Tuesday. He made clear that means the Bidens.

Conservative Trump supporters are eager to reprise the successful attacks on Kerry and Clinton, and believe public testimony will further the effort to derail Biden.

“Biden can expect Trump and outside groups to deliver the same experience that wrecked Clinton’s and Kerry’s campaigns,” said Steve Bannon, chief strategist of Trump’s 2016 campaign and an architect of the effort to take down Hillary Clinton. “Isolate and amplify the most damaging charge against the strongest Democratic candidate and hammer into voters’ minds until Election Day.”

Caucus Voters

That strategy is already in play. After Bondi’s presentation on Monday, Iowa Republican Senator Joni Ernst said that she’s “really interested to see how this discussion today informs and influences the Iowa caucus voters, those Democratic caucus-goers.”

Biden’s campaign has worked vigilantly to avoid that outcome. On Jan. 20, it sent media outlets a three-page memo preemptively admonishing reporters and editors for giving any credence to Trump’s charges and going so far as to suggest specific language that outlets should use when writing about Ukraine and the Bidens.

After Ernst made her remarks, Biden told reporters in Muscatine, Iowa, that it made clear that Republicans are using the impeachment trial to damage his bid for the Democratic nomination.

“She spilled the beans,” Biden said. “The whole impeachment trial for Trump is just a political hit job to try to smear me because he is scared to death of running against me.”

There is little evidence to date that Democrats have been influenced by the attacks on Biden. Experts say that may stem from Democrats’ internal political compass.

Stable Base

“Well-known figures like Biden have a stable base of voter support,” says Lynn Vavreck, a UCLA political scientist and co-author of “Identity Crisis,” a detailed study of what influenced voter sentiment in the 2016 presidential election. “The fact that Trump is the primary messenger for these attacks also triggers party loyalty.”

Brian Fallon, a Democratic strategist who worked on Hillary Clinton’s 2016 campaign, says that there are two signal differences between Biden’s current situation and Clinton’s experience in the last campaign. The first is the public profile of the two politicians.

“I think there are systemic factors that make the Biden attacks less likely to take hold,” he says. “The email saga in some ways corroborated prejudices that a lot of voters already had against Hillary, whereas Hunter Biden introduced an unfounded corruption allegation that has never been part of Joe Biden’s story and doesn’t reinforce a negative that’s already in the public eye.”

The second difference, Fallon said, is the sophistication that’s developed among Democratic and independent voters, as well as the news media, after three years in a Trump presidency.

Fallon still thinks Democrats should do all they can to avoid having Hunter Biden testify. “But let’s presume it happens,” he says. “Even in a worst case scenario, yes, it’ll be talked about and covered in the media. But I don’t think it’s likely that a reasonable person will accept Trump’s premise. They’ll see it for what it is: a Hail Mary attempt to muddy the waters in his impeachment trial.”

One potential difficulty for Biden is that while Democrats may not trust the messenger, they’re about to choose a standard-bearer and could end up deciding that nominating him isn’t worth the risk.

“We really don’t know to what extent people are inoculated at this point,” Shrum said. He adds that strong testimony from Hunter Biden could even produce a backlash that redounds to the benefit of his father’s campaign.

While no one can forecast the outcome, Shrum says, there’s one certainty to be drawn if the Senate subpoenas Hunter Biden. “If they force him to testify,” he says, “they’ll keep talking about it, and talking about it, and talking about it.”

To contact the reporter on this story: Joshua Green in Washington at jgreen120@bloomberg.net

To contact the editors responsible for this story: Wendy Benjaminson at wbenjaminson@bloomberg.net, Joe Sobczyk

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