Several GOP senators are refusing to commit to subpoena John Bolton, following the former national security advisor’s letter detailing his willingness to comply in the event he is called to the Senate impeachment trial.

Bolton said in a statement on Monday he will testify if the Senate issues a subpoena for his testimony.

“Accordingly, since my testimony is once again at issue, I have had to resolve the serious competing issues as best I could, based on careful consideration and study,” Bolton said in a statement.

“I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify,” he continued.

The statement served as a boost for Senate Minority Leader Chuck Schumer (D-CA), who has been vying for at least four Republicans to cross over, giving Democrats a simple majority to pass an organizing resolution that satisfies their immediate demands for witnesses.

“A simple majority is all it takes to ensure that the Senate issues a subpoena for these witness if only four Republicans decide that Mr. Bolton and the three other witnesses ought to be heard, they will be heard because every Democrat will vote to hear them,” Schumer said on the Senate floor, accusing GOP senators who oppose his testimony of engaging in a “cover-up.”

He said:

Given Mister Bolton’s lawyers have stated has new and relevant information to share, if any Senate Republican opposes issuing subpoenas to the four witnesses and documents we’ve requested, they would make it absolutely clear they are participating in cover-up.

Several GOP senators are skeptical, refusing to commit to subpoena Bolton until parameters of the trial are set. Sens. Susan Collins (R-ME) and Lisa Murkowski (R-AK), for instance, believe the call for Bolton to testify is premature and have called for the Senate to follow the precedent of the Clinton impeachment trial, holding off on considering additional witnesses until after opening arguments.

“I don’t think there is any decision on Bolton because we don’t have articles,” Murkowski told reporters.

“I think it’s difficult to decide in isolation before we have heard the opening statements. … There are a number of witnesses that may well be appropriate for Stage 3, of which he would certainly be one,” Collins said, according to Axios.

“I would like to be able to hear from John Bolton. What the process is to make that happen, I don’t have an answer for you,” Sen. Mitt Romney (R-UT) said of the demands for Bolton to testify.