Senate Democrats have a problem: They want key witnesses like former national security advisor John Bolton to testify in the impeachment trial, but they’re four votes short of a Senate majority. So they’re looking for other ways to get what they need—including, reportedly, giving the Republicans a key concession: Testimony from a Biden.

The Washington Post reported late Tuesday that a “small group of Democratic senators” are privately mulling a potential deal to subpoena either Vice President and current Democratic presidential candidate Joe Biden or his son, Hunter—the ex-Burisma board member whom President Donald Trump and his allies pushed the Ukrainian government to investigate—to testify.

The idea’s origins lie in the Republican caucus, where Sen. Ted Cruz reportedly suggested the trade in a private GOP meeting last week. The Democrats’ tentative approach to a trade, however, indicates that it’s not as far-fetched as it might seem.

The rules for the impeachment trial passed by the Senate on early Wednesday morning allow for the possibility of calling witnesses later in the trial, although the witnesses would initially be deposed before the Senate decides whether or not to allow them to testify.

As the Republicans have a 53-47 majority in the Senate, any push by the Democrats will require some Republican support. All 11 proposed Democratic amendments to the rules, including one that would have subpoenaed Bolton, were defeated during Tuesday’s 13-hour session.

READ: Here we go: Trump’s impeachment trial just kicked off in the Senate

At least one Biden ally, Delaware Sen. Chris Coons, has been publicly receptive of the idea of having the longtime former senator testify. “I can’t imagine a person more comfortable in the well of the Senate than a man who spent 36 years here as a United States senator,” Coons told the Post.

Other Democrats maintained that Hunter Biden was irrelevant to the impeachment trial, although none exactly shot the idea down. “If there are four Republicans who open the conversation about witnesses, then I assume there will be a negotiation that follows — but I can’t tell you where it goes,” Senate Democratic whip Dick Durbin told the Post.

The first day of the trial ran well into the night, and tempers over the question of whether to call Bolton ran high. After a heated exchange between House “manager” Rep. Jerry Nadler and White House counsel Pat Cipollone, Chief Justice John Roberts, who’s presiding over the trial, admonished the managers and Trump’s defense team. “Those addressing the Senate should remember where they are,” Roberts said.

Oral arguments are set to begin on Wednesday afternoon.

Cover: Former National security adviser John Bolton gestures while speakings at the Center for Strategic and International Studies in Washington, Monday, Sept. 30, 2019. (AP Photo/Pablo Martinez Monsivais)