Brushing off calls by Democrats to make an agreement now on witnesses, Senate Republicans made clear on Tuesday that they have enough support within GOP ranks to open a Senate impeachment trial of President Donald Trump, whenever House Democrats decide to physically bring the actual impeachment articles to the other side of the U.S. Capitol.

"We have the votes," Senate Majority Leader Mitch McConnell told reporters after a weekly lunch gathering of GOP Senators, as Republicans say they will offer a resolution based on a bipartisan agreement which was used to begin the Clinton impeachment trial in 1999.

The plan would allow for opening arguments from House prosecutors, from the President's defense team, and then written questions by Senators - who are not allowed to engage in debate while the trial is in session.

"At that point, during the impeachment trial, the appropriateness of calling witnesses was addressed," McConnell said.

"Obviously that it one of the most contentious part of these proceedings," McConnell said about the question of trial witnesses, which will be determined simply by majority vote.

"Fifty one Senators determine what we do," McConnell added in a reminder that there is an inherent risk that a trial could get away from GOP control, if a handful of Republicans were to break ranks, and demand to hear from certain witnesses.

GOP Senators said they had no problems following the outlines of the initial trial rules which were used 21 years ago.

"What's good for President Clinton is good for President Trump," said Sen. John Barrasso (R-WY).

Democrats grumbled about the decision, as they have tried to pressure the GOP to also make an agreement on witnesses for the trial.

"What are the Republicans afraid of?" asked Sen. Debbie Stabenow (D-MI).

Senate Democratic Leader Charles Schumer again tried to make the case that Republicans were ready to hold back information from the public.

At this point, no one knows when the trial will actually begin, waiting to see when House Speaker Nancy Pelosi will send the impeachment papers to the Senate.

"It is a rule of impeachment in the Senate that we must receive the papers," McConnell told reporters, as until then, the trial cannot start.