As the Senate’s showdown over whether to call witnesses in President Trump’s impeachment trial looms, the president’s allies are making an increasingly blatant threat: If Democrats get to call witnesses, they really, truly will subpoena Hunter Biden.

“If there’s gonna be one witness, there’ll be more. And there’s 53 Republican votes to call Hunter Biden,” Sen. Lindsey Graham (R-SC) asserted Wednesday evening. Trump lawyer Jay Sekulow also told the Senate that if there are any witnesses, Hunter Biden would be one of the four he would call (along with Joe Biden, Adam Schiff, and the whistleblower).

The threat to subpoena Hunter has been hanging over impeachment proceedings for months now — even though Hunter has no relevant knowledge of whether President Trump abused his power or obstructed Congress, the two charges the president faces. Calling Hunter would be partly meant to change the subject, to put the Bidens on trial rather than Trump. It would also be an attempt to extract political payback from Democrats for daring to try to hold Trump accountable.

For now, though, it’s mainly a threat designed to get Democrats to back down. Senate Majority Leader Mitch McConnell has privately referred to the witness issue as “mutually assured destruction.” In large part, that’s because of Hunter. Republicans believe that any testimony from Hunter could be tremendously ugly for Joe Biden’s presidential campaign — so ugly that Democrats would never permit it to happen.

It’s not clear they’re wrong. Minority Leader Chuck Schumer has adamantly opposed any hypothetical “deal” on witnesses that would include testimony from either Biden. And a potential problem here is that even Republicans who say they’re leaning in favor of calling witnesses, like Sen. Susan Collins (R-ME), tend to also stress that they would let “each side” pick witnesses.

In theory, Collins’s position is about fairness to the president. In practice — so long as the Senate doesn’t agree to assess every individual witness to make sure he or she is relevant — it would likely mean calling Hunter. So though theoretically Collins and Democrats both seem to want witnesses, it’s not clear they could reach a specific agreement.

Why anyone’s talking about calling Hunter Biden in the first place

The Ukraine scandal at its heart is about Trump’s attempts to dirty up Joe Biden politically. He urged Ukrainian President Volodymyr Zelensky to “look into” the Bidens, referencing “Biden’s son.” And his administration withheld a White House meeting and hundreds of millions of dollars of military aid from the Ukrainians unless they let it happen.

The background here is that Hunter Biden had a well-compensated position on the board of a Ukrainian gas company, Burisma — a company that has been accused of corruption. Hunter had no expertise in the field or the region, and his main qualification for the gig appeared to be that his father was vice president and involved in Ukraine policy at the time, which doesn’t exactly look great.

However, accepting a high-paying job isn’t illegal. And Trump’s unfounded conspiracy theory — that Joe Biden forced out Ukraine’s prosecutor general Viktor Shokin to protect Burisma and his son from an investigation — seems to make little sense, as it was the consensus position of the US government and Western institutions like the International Monetary Fund to oust Shokin due to his own corruption and ineffectiveness.

One of the arguments Trump’s team keeps offering in his defense, though, is that Hunter Biden really was corrupt, and that therefore, the president was well in his rights to request an investigation into that corruption. So, their argument is, calling Hunter to testify will help Trump determine whether this is true.

Democrats have insisted that what matters here isn’t what Hunter Biden did, it’s what Trump knew and did. Using the impeachment trial to grill Hunter under oath would in fact be giving Trump what he wanted all along. Still, Hunter’s name has come up so often that Sen. Joe Manchin (D-WV), for one, has opined that he is indeed a “relevant witness.”

The political calculations around the Hunter Biden threat

Though McConnell’s “mutually assured destruction” metaphor for the witness issue is evocative, a better one comes from the movie The Untouchables.

“They pull a knife, you pull a gun. He sends one of yours to the hospital, you send one of his to the morgue. That’s the Chicago way,” Officer Malone says, in explaining how to win a showdown with Al Capone.

This is how Trump operates on many fronts. He pursues feuds for years (including, in some cases, after his enemy has died.) He sought and is in fact still seeking payback against FBI officials who dared to have him investigated. He threatened that he would strike back against Iran “perhaps in a disproportionate manner.”

Here, his play is: If Democrats dare to investigate his abuses of power, he’ll try to make it so politically painful for their potential 2020 nominee that the party will live to regret it. And that would be the true purpose of subpoenaing Hunter.

Some Trump critics have deemed this a bluff and urge the Bidens to call it. “Joe Biden and Hunter Biden should announce tonight that they’d be delighted to come testify before the Senate trial,” historian Kevin Kruse tweeted. Journalist Elizabeth Drew wrote a New York Times op-ed headlined “Why having Hunter Biden testify would be bad for Trump.”

Yet Biden himself said last week he “wants no part” in the trial, saying his testimony would “turn it into a farce.” And leading Democrats and the Biden campaign have been very resistant to any idea of a witness swap — likely for good reason.

Volunteering to testify means volunteering to be grilled under oath by Trump’s legal team. Even for Joe Biden, for whom there’s no credible evidence of any wrongdoing here, that’s an enormously risky move, bordering on political malpractice. The former vice president has not exactly sought out high-stakes, adversarial interviews during his campaign this year — let alone interviews with his potential opponent’s lawyers where there could be legal consequences for any misstatements he makes.

Testimony from Hunter would be even riskier. Hunter has had a troubled few decades; he’s struggled with addiction, the law, and his personal life. He also has a history of accepting large sums from questionable foreign sources — including, in one case, a “large diamond” from a Chinese energy tycoon. (“I knew it wasn’t a good idea to take it,” Hunter later told the New Yorker. “I just felt like it was weird.”) It’s far from clear how he’d hold up in a setting like this. How confident are Democrats, really, that something scandalous won’t come out?

In return for taking these enormous political risks (and rewarding Trump’s effort to dirty up the Bidens), what would Democrats get in return? Testimony from John Bolton? He’ll tell his story soon in his book, anyway, and if the key aspects of it have already leaked, it’s clearly not enough to change the outcome of the trial. Testimony from Mick Mulvaney? He will be a hostile witness trying to cover for Trump as best he can.

So long as Biden remains a potential general election nominee for Democrats, they will be wary of making a move that could badly hurt his campaign. Accordingly, the messaging from Schumer and other Democrats has been that, since Republicans have the majority, it’s up to them to call whatever witnesses they want — they don’t “need” Democratic votes, and certainly can’t count on them for any proposal involving testimony from a Biden.

McConnell’s preference is still to have a clean vote in which at least 50 of the 53 Republicans unite to block the possibility of any witness testimony. But if the votes aren’t there for that, the Hunter threat provides another opportunity — a possible poison pill that could scuttle any witness deal.