A group of Republican senators said they want a chance to consider witness testimony in President Trump’s impeachment trial, raising the chances they’ll summon former national security adviser John Bolton.

Bolton, who left the White House in September — Trump said he was fired, while Bolton said he quit — said last month he is willing to testify in the Senate impeachment trial if called by lawmakers, although Trump indicated he might use executive privilege to block him from speaking.

Susan Collins of Maine, Lisa Murkowski of Alaska, and Mitt Romney of Utah have all signaled they may want to hear Bolton’s testimony.

Sen. Lamar Alexander, a Tennessee Republican who is retiring after his term ends, said he might support a vote to call witnesses but did not indicate who should be on the list.

Democrats need four Republican votes to win the majority needed to call a witness.

Alexander, Collins, Murkowski, and Romney said they want to hear arguments from the House impeachment managers, as well as the White House defense team, before deciding whether they should call witnesses.

“We should have a vote on calling witnesses or asking for documents after we have heard the case,” Alexander told the Washington Examiner on Tuesday. “That could mean witnesses, or it could not.”

Bolton tops a list of four witnesses Democrats are seeking, but the party also wants to call acting White House chief of staff Mick Mulvaney and two other administration officials.

Democratic lawmakers believe Bolton can bolster their charges that Trump abused the power of his office by withholding critical security aid from Ukraine for his own personal benefit.

Bolton, during his time as national security adviser, objected to efforts by Gordon Sondland, the U.S. ambassador to the European Union; Mick Mulvaney, the acting White House chief of staff; and Trump’s personal lawyer Rudy Giuliani to try to coerce Ukraine government officials to investigate former Vice President Joe Biden and the Democrats.

While Bolton sought a ruling from a federal court when the House called him to testify in their impeachment investigation last year, he said recently he’s willing to serve as a witness in the Senate trial.

Trump last week indicated he’d invoke executive privilege to block Bolton’s testimony.

Sen. Lindsey Graham, a South Carolina Republican and Trump ally, said the White House would seek to summon their own witnesses. That list could include Hunter Biden, the son of Joe Biden, who held a lucrative position on a Ukraine gas company while his father was vice president.

“I don’t imagine any scenario where the White House gets shut out when it comes to witnesses,” Graham said.