With Trump finally facing his impeachment trial, this  promises to be a defining moment for both men, They started their relationship unevenly three years ago when Trump stunned Washington with his sweep to power but have since fallen into an easy partnership that will be put to its biggest test.

The leader of the Republican-majority Senate has already put his imprint on virtually every aspect of the upcoming trial. He corralled the GOP senators  behind his strategy to brush back Democratic demands for new witnesses and testimony. On Monday, McConnell and Senate Republicans were trying to decide whether to include a motion to simply dismiss the charges against Trump outright, as the president wants, in the organizing resolution for the trial, according to a person familiar with the matter but unauthorized to discuss it.

The Kentucky Republican is working hand in hand with the White House. He doesn't pretend to be an impartial arbiter.

"The House has done enough damage," McConnell said Monday as he opened the chamber. "The Senate is ready to fulfill our duty."

As the Senate is about to convene for the landmark undertaking, only the third presidential impeachment trial in American history, perhaps no one is more important to Trump's defense than the Republican leader.

The Democratic-run House is set to transmit the articles of impeachment to the Senate in a matter of days. Trump faces two charges  approved by the House. First, that he abused power by pushing Ukraine to investigate his Democratic political rival Joe Biden, holding back U.S. military funds to the country as leverage. And second, that he then obstructed Congress by blocking witnesses and testimony in the House probe.

The challenge for McConnell will be to balance Trump's appetite for full vindication, accompanied by humiliation of Democrats, with a more measured trial that fits the legal expectations of the Constitution and won't expose Senate Republicans to a spectacle that could hurt them in elections.

"The president and Senator McConnell have learned to trust each other's judgment." said Josh Holmes, a former top McConnell aide who remains close to the leader. "They've been through an awful lot over three years."

Republicans hold the majority in the Senate, 53-47, and there is nowhere near the 67 votes needed for Trump's removal.

The president has given mixed messages about what he wants in a trial — first suggesting calling witnesses including Speaker Nancy Pelosi, House Intelligence Chairman Adam Schiff and the still anonymous government whistleblower, whose complaint about Trump's July phone call with Ukraine sparked the impeachment inquiry.

But over the weekend Trump said the Senate should simply dismiss the case against him, rather than legitimize the House charges by sending them for trial. It was an extraordinary suggestion that now appears to be under consideration.

As Pelosi prepared to release her hold on the charges, Trump tweeted yet again on Monday that the House impeachment inquiry "was the most unfair witch-hunt in the history of Congress!" Making it personal, McConnell is calling the holdup Pelosi's "one-woman blockade."

The president continues to review his options, according to one senior White House official who spoke on condition of anonymity to discuss internal deliberations.

The person described the president's talk of dismissal as simply covering the possibilities as the White House continues to work closely with Republicans on Capitol Hill. Simply dismissing the charges against Trump is unlikely, though Republicans are circulating a proposed resolution.

McConnell, who has gained the president's trust in Senate matters, now needs to deliver.

The two sparred early in Trump's presidency, most sharply when McConnell's Senate GOP was unable to repeal the Affordable Care Act, a signature Trump campaign promise.

But once the GOP leader muscled through the confirmations of Trump's Supreme Court nominees, Neil Gorsuch and Brett Kavanaugh, the relationship warmed. The two men talk regularly.

The leader's proximity to Trump helps him back home, where he is up for reelection in the Bluegrass State this year alongside Trump. His likely Democratic opponent will be former Marine Corps officer Amy McGrath, who flew combat missions in Iraq and Afghanistan.

As the House launched the impeachment inquiry, McConnell was able to convince Trump not to tweet against Senate Republicans who showed signs of straying and instead focus attention on the House.

McConnell's goal during the House proceeding was to create a party-line outcome that would diminish the case as it came to the Senate, Holmes said.

The groundwork for the Senate trial was laid months ago as McConnell built GOP support for modeling it partly on rules devised for President Bill Clinton's impeachment trial in 1999. That would mean starting the proceedings and voting only later on hearing new testimony.

That allowed McConnell to turn back Democrats' demands for agreement on new testimony, particularly from former White House national security adviser John Bolton, who has indicated he will defy Trump's orders and appear if subpoenaed.

Democrats still hope to force votes on Bolton and other witnesses they say can provide new evidence for the case against Trump. McConnell is trying to prevent any votes that will prolong the trial and split his party, particularly vulnerable senators up for reelection in 2020.

Senate Minority Leader Chuck Schumer warned Monday that failing to call new witnesses and testimony would turn the Senate trial into a "farce."'

One Republican, Sen. Susan Collins of Maine, is working with GOP colleagues on a process that would allow them to hear more testimony, as Democrats want. McConnell is trying to give her and the others room to see if they can make a deal, the person familiar with the matter said.

Trump has yet to decide on his legal team and whether it will grow to include some of his fiercest defenders from the ranks of House Republicans.

Alan Dershowitz, the former Harvard University professor whom the president is considering adding to his defense team, said that the president has a "good team" with White House Counsel Pat Cipollone and personal attorney Jay Sekulow.

Dershowitz said Monday that no final decision had been made by the president.

Trump is scheduled to attend the annual World Economic Forum in Davos, Switzerland, early next week - just as the Senate trial is expected to be underway in Washington.

The opening will be in McConnell's hands.