Washington (CNN) A handful of key Republican senators dodged questions Monday on whether they'd support a subpoena for former White House national security adviser John Bolton to testify in the Senate impeachment trial of President Donald Trump, even as most of their GOP colleagues expressed strong resistance to calling him to come before the body.

The GOP senators hold the key to whether Bolton will offer his much-anticipated testimony , since 51 senators can vote to subpoena witnesses in an impeachment trial, meaning that the 47 Senate Democrats would need to pick up four GOP defectors.

On Monday, Republicans who are expected to face tough battles this year to hold on to their Senate seats, including Sens. Cory Gardner of Colorado and Martha McSally of Arizona, would not respond directly when asked if they'd support a subpoena for Bolton. Senators who have broken with Trump at times, including Lisa Murkowski of Alaska and Susan Collins of Maine, who could face a tough race in her state this fall, said it was too early to say whether the Senate should issue a subpoena for Bolton.

And an occasional Trump critic, Sen. Mitt Romney of Utah, went the furthest by saying "of course" he wants to hear from Bolton, though he stopped short of saying he'd vote for a subpoena, saying he wants to know what the process is first.

"I'd like to hear what he has to say," Romney told CNN.

Bolton issued a statement Monday after the courts did not rule whether he would be compelled to testify during the House's impeachment proceedings, saying he was trying to meet his "obligations both as a citizen and as former national security adviser."

Bolton's testimony could be revealing because he described the interactions between top Trump aides and Ukrainian officials as a "drug deal," a colorful and damning analogy that he's never fully explained. Bolton's attorney also has said that his client -- who had many firsthand conversations with Trump -- has knowledge of matters that have not yet been presented during the impeachment proceedings.

Still, the news that Bolton is willing to testify in a trial if he is subpoenaed did not seem to move the needle with most Republicans, particularly in the Senate leadership, with one Republican senator after another arguing that the Senate should consider the case outlined by the House when it voted to approve articles of impeachment over the Ukraine scandal, and that it is not the Senate's job to call additional witnesses or try to compel testimony.

Asked if he would like to hear testimony from Bolton, Sen. Todd Young, an Indiana Republican who is a member of the GOP leadership, replied, "I wonder if the House would. They're in charge of impeachment, as you know. We are in charge of trying the articles of impeachment. I think it would set a very bad precedent if the Senate made it a common practice to try and to improve upon defective articles of impeachment."

Only a handful of Republican senators expressed any openness to hearing from Bolton during a Senate trial, and even those who did still did not call for him to be subpoenaed.

Sen. John Thune of South Dakota, the No. 2 Senate Republican, said when asked if he would like Bolton's testimony to be part of the trial record: "If it was supposed to be, I think they would have done it in the House. To me, they could have subpoenaed him. They didn't."

"Seems to be the record the Senate is being asked to pass judgment on is the record the House put together in the articles. And that's what I think this exercise in the Senate ought to be about," Thune said.

Sen. Marco Rubio of Florida, a member of the Senate Intelligence Committee, told CNN he would not vote for a subpoena of Bolton because it was the House of Representatives' job to get his testimony.

"I wouldn't because ... I believe you should be constrained by the information that those articles are based on. ... If the House wants to start a new impeachment inquiry or pull it back and add additional elements to it, that's their choice to make," he said.

Sen. Lindsey Graham of South Carolina, an ally of the President, echoed that sentiment, saying, "I'm ready to go. If they wanted to call Bolton, they should have called Bolton," in a reference to the Democrat-controlled House.

Democrats have insisted that a deal first be cut on witnesses and documents before the ground rules of the trial are set, a condition Senate Majority Leader Mitch McConnell continued to reject Monday. The Kentucky Republican said witnesses and documents should be a question that senators consider later.

Yet it's unclear how senators will vote at a later point or demand Bolton's testimony.

Collins, the Maine Republican facing reelection in a state Hillary Clinton won in 2016, expressed interest in hearing from Bolton during a later stage in the trial but showed no inclination to push to bring him in before that point. Collins made clear she views the trial in three stages: presentations, then senator questions, then discussion of whether to have witnesses.

"There are a number of witnesses that may well be appropriate for the Stage 3, of which he would certainly be one," Collins said, adding, "But it's very difficult to decide that until we go through the first two stages and look at all of the witnesses that each side would like to have if we get to that stage."

Murkowski, another moderate Republican who will be closely watched as a possible swing vote during a trial, would not directly answer when asked by CNN if she wants to hear from Bolton.

"I want to get to the first step. The first step is trying to get articles of impeachment, which we haven't gotten yet. So there's a lot of people who want to hear a lot of things, but we've got to get to the first step first," she said.

McSally dodged questions on whether she would support a Bolton subpoena, saying, "We will work through it."

Gardner responded by attempting to turn the subject toward House Speaker Nancy Pelosi, a California Democrat, saying, "I know you guys want to have a trial by Twitter -- but is Nancy Pelosi going to follow the Constitution? ... Until she has the articles sent over, there is no trial."

The House still has not yet sent articles of impeachment over to the Senate, creating uncertainty over when a trial will start, while Senate Democratic leader Chuck Schumer of New York and McConnell continue to clash over how to set the ground rules for organizing a trial. Schumer reiterated his call for witnesses, including Bolton, with a warning to Republicans.

"Make no mistake: There will be votes on whether to call each of the four witnesses we proposed and subpoena the documents we identified," Schumer said, adding, "My colleagues on the other side of the aisle: Your constituents and the voice of history is watching."

McConnell has rejected Democratic demands to require that the ground rules of the trial include testimony from Bolton and three other witnesses, along with documents withheld by the White House. The majority leader says such decisions about witnesses and documents should be made after opening arguments, though Democrats fear that's an attempt to prevent any witnesses from testifying.

In remarks on the Senate floor, McConnell did not mention Bolton but reiterated what he has said before, that decisions about witnesses should be dealt with later, during the trial.

"The Senate has a unanimous bipartisan precedent for when to handle mid-trial questions such as witnesses. In the middle of the trial," McConnell said, a reference to how the Senate dealt with the impeachment trial of former Democratic President Bill Clinton.

The news that Bolton is willing to testify is unlikely to change McConnell's strategy over the trial, according to multiple GOP sources familiar with the matter. "Not at all," one source said, and McConnell plans to reiterate his view to Senate Republicans during a closed-door lunch on Tuesday, another source said.