House Speaker Nancy Pelosi is continuing demands that Senate Majority Leader Mitch McConnell release plans on how President Donald Trump’s impeachment trial will be conducted.

Pelosi’s move comes on the heels of news that McConnell already has enough Senate votes to block Democrat requests for additional witnesses during a Trump trial.

Two key GOP senators – Susan Collins of Maine and Lisa Murkowski of Alaska – are backing McConnell's insistence that Trump's impeachment trial follow that of Bill Clinton's in 1999 and defer the process of calling additional witnesses.

President Bill Clinton was impeached on perjury and obstruction of Congress charges on Dec. 19, 1998. He was acquitted by the Senate on Feb. 12, 1999. Fourteen senators from that trial still remain in office. Here's how they voted. Susan Collins, R-Maine. Not guilty on both counts Michael D. Crapo, R-Idaho. Guilty on both counts Richard Durbin, D-Illinois. Not guilty on both counts Mike Enzi, R-Wyoming. Guilty on both counts Dianne Feinstein, D-California. Not guilty on both counts Charles Grassley, R-Iow

Senate Minority Leader Chuck Schumer (D-N.Y.) and other House Democrats have been demanding additional witnesses be called during a Trump trial. Senate scheduling of the trial has been stalled on Capitol Hill for weeks.

Pelosi reiterated her demands in a letter sent to her Democratic colleagues Tuesday evening.

Read Pelosi’s letter:

McConnell also told reporters Tuesday he has enough votes to set the ground rules for a trial without Democratic support, according to Conservative Angle.

Despite Democrats' professed sense of urgency in passing House impeachment articles against the president last month, Pelosi has delayed sending the charges over to the Senate and refused to name the House managers who would handle the trial until Senate GOP leaders meet her demands.

On Monday, former White House national security adviser John Bolton said he would be willing to testify in Trump’s trial if subpoenaed by the Senate.

Donald Trump has become the third American president to be impeached. Trump has been charged with abuse of power and obstruction of Congress. Led by Democrats, the U.S. House of Representatives passed the articles of impeachment on Dec. 18, 2019. Trump will face trial in the GOP-controlled Senate in 2020, a presidential election year. Presidents Andrew Johnson and Bill Clinton were also impeached. Neither was removed from office.

Republicans control 53 Senate seats and Democrats have 45, plus the support of two independents. Schumer needs at least four Republicans on his side to pass a resolution for the trial meeting his demands.

Schumer wants Bolton and acting White House chief of staff Mick Mulvaney; Robert Blair, a Mulvaney adviser; and Michael Duffey, an Office of Management and Budget associate director, to testify at Trump’s trial.

Trump impeachment trial: John Bolton says he will testify if subpoenaed

In her letter, Pelosi circulated a document from Schumer arguing the witnesses called in the Senate trial had already testified under oath to a grand jury, while witnesses sought by Democrats refused to testify during House impeachment hearings.

»RELATED: More senators breaking ranks from their parties on impeachment

Trump is only the third sitting president in American history to be impeached, joining Andrew Johnson and Clinton.