These are the circumstances that could actually force Trump to resign: history professor

It's highly unlikely -- though not impossible -- that President Donald Trump could step down to avoid being thrown out of office.

House Democrats have launched a formal impeachment inquiry that's already gathered new public support, but Trump's history suggests he'll fight to the bitter end -- no matter what it costs the nation, according to CNN political analyst Julian Zelizer, a professor of history and public affairs at Princeton University.

"The prospects of Trump ending his career rather than forcing others to do it for him -- either Congress or the electorate -- seem remote," Zelizer argued. "Trump loves a fight and seems to be revved up by the prospect of taking on the Democratic House."

Trump loves to feel like he's punching back, and he seems to think he's entering the impeachment fight with the full support of the Republican Party.

"With memories of the 1998 midterms, when the electorate punished the GOP for impeaching President Bill Clinton, Trump is betting that voters will turn against the Democrats rather than him, putting him in an even stronger position by Election Day," Zelizer wrote.

But some observers believe he could eventually be forced to abandon the fight if resignation is all that's left to keep him from being prosecuted upon leaving office.

"If Senate Majority Leader Mitch McConnell met with Trump to deliver the news that his colleagues would not support the administration should a trial prove that articles of impeachment were sound, the President would suddenly be the one sitting on the other end of the boardroom table about to be told, 'You're fired!'" Zelizer wrote.

Right now, congressional Republicans have signaled they support the president, but eventually the pressure to turn on Trump may become too great, Zelizer wrote.

"The power of partisanship on which Trump has depended might turn against him if the party no longer sees him as being in its best interests," Zelizer wrote. "More than good conscience or realism, the loss of Republican support in the Senate will be the only development that can leave President Trump standing alone, with no choice but to step down."