The Democratic-controlled House of Representatives last month voted to impeach Trump for abuse of power in pressuring Ukraine to investigate a political rival and for obstructing Congress' investigation.
And it's one measure of the grip Trump has over the party.
Congressional investigators believe Bolton objected to Trump's decision to delay $390 million in military aid to Ukraine and could elaborate on that, a Senate aide told Reuters.
"The desire is to get a commitment from the Senate that they're going to have a fair trial", he said. Senators who have broken with Trump at times, including Lisa Murkowski of Alaska and Susan Collins of ME, 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. He declined further comment.
Senate majority leader Mitch McConnell, a Republican, said on Friday the Senate can not proceed with a trial under its rules until it receives them.
If the Democrats had any hope of impeaching President Donald Trump's impeachment process, they would need help from Republicans like Senator Cory Gardner.
"If she continues to refuse to send the articles as required by the Constitution, I would work with Senator McConnell to change the rules of the Senate so we could start the trial without her, if necessary", Graham said on Fox News' Sunday Morning Features.
Democrats would have to take four seats to win back the Senate - or win three seats and the White House to have a pioneering vice president.
A conviction on the actual impeachment charges requires a two-thirds majority vote in the Senate. He also chaired the National Republican Senatorial Committee in 2018, helping to develop a medium-term strategy that enlarged the majority of heads of government even if they lost control of the House of Representatives.
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.
"Let the House come over and prove their case and let's vote", Shelby said.
Bolton's willingness aside, his subpoena and testimony are not a given.
"The Senate can not be complicit in the President's cover-up".
The earliest the House could take any action would be on Tuesday when it reconvenes, but top Democrats have given no sign they are set to move this week.
"But after a bipartisan vote against the articles in the House, and with the public opposed to the Democrats' partisan games, Pelosi has changed her tune".
"It's beyond me how the speaker and Democratic leader in the Senate think withholding the articles of impeachment and not sending them over gives them leverage", McConnell said last month.
The White House claimed executive privilege to prevent the four - Bolton, White House Chief of Staff Mick Mulvaney, Trump assistant Robert Blair, and budget official Michael Duffey - from testifying to the House impeachment investigation.
House Speaker Nancy Pelosi, however, has not yet sent the articles to the Senate, where the president will be tried, as Democrats have sought to pressure Republicans to call witnesses.
