Before the Senate impeachment trial against President Trump kicks off Tuesday, former House Speaker Newt Gingrich laid out the two main reasons he believes Nancy Pelosi, D- Calif., decided to pursue impeachment.

“One is that the passion on the left is so enormous, that she didn’t have any choice,” Gingrich told “Fox & Friends,” “And that to not have moved it would have led to such a rebellion in her party that she had to go ahead and try to do it."

SENATE IMPEACHMENT TRIAL SET TO BEGIN AS TRUMP ADDS LAST-MINUTE REINFORCEMENTS TO DEFENSE TEAM

After an opening salvo of back-and-forth arguments from President Trump's attorneys and Democrats' impeachment managers on Monday, Senate impeachment trial proceedings are set to begin at 1 p.m. ET on Tuesday with the expectation they will stretch well into a wild night on Capitol Hill -- even as key procedural questions, including the timeline for debate and whether additional witnesses will testify, remain undecided and hotly contentious.

In a surprise move Monday night, a detachment of high-profile House Republicans announced that they would formally join the president's legal team, including Reps. Doug Collins, Mike Johnson, Jim Jordan, Debbie Lesko, Mark Meadows, John Ratcliffe, Elise Stefanik and Lee Zeldin.

The last-minute show of force underscored the fluid nature of the Senate trial, which is also set to feature full-throated arguments against impeachment from constitutional scholar Alan Dershowitz and Bill Clinton independent counsel Ken Starr.

Gingrich said the other more "cynical" factor behind the impeachment is the Democrats' belief that they will “scar Trump” enough to defeat him in November. But Gingrich said he believes it's having the opposite effect.

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“I think, in fact, they vaccinated him and he’s even more likely to win now than he was before this [impeachment] started,” Gingrich said, applauding the president for traveling to the World Economic Forum in Davos, Switzerland Tuesday as a "brilliant" counter-programming strategy against the impeachment trial.

Fox News’ Gregg Re contributed to this report.