The Note: Sanders vs. Warren fight tests both candidates at only Iowa debate

The TAKE with Rick Klein

Will they go there -- face to face?

Six candidates -- all of them white -- take the stage Tuesday night for the first and only Iowa debate this election cycle with the race entering an unusual -- and what looks like a more aggressive -- phase.

While centrists have been worried about the rise of the left, two progressive champions are primed to have a debate with each other that they've both said they didn't want to have. But that's up to them.

The non-aggression pact between Sens. Bernie Sanders and Elizabeth Warren has been a prominent feature of the previous six Democratic debates. That will mean nothing if they choose this moment to say publicly what their campaigns are saying about each other privately -- marking perhaps the biggest strategic question facing the major Democratic candidates in the final 20 days before the caucuses.

It's hard not to go there now. Warren issued a statement late Monday essentially confirming a CNN report that when she and Sanders spoke about her candidacy more than a year ago, "he disagreed" when she said she thought a woman could win the presidency.

Warren added that she has "no interest" in talking further about that private conversation, which she called "punditry." But, with Sanders denying it and the debate set to air on CNN, good luck with that.

Sanders and Warren represent different strands of a powerful movement. Only one, of course, can win the nomination, and issues of socialism, sexism and elitism could play into who is best positioned to own the left.

Sanders' supporters are a dedicated bunch, while anti-Sanders sentiments continue to run deep in establishment Democratic circles. Does Warren have a plan for all this?

The RUNDOWN with MaryAlice Parks

The country now has a likely start date to the Senate impeachment trial of President Donald Trump. And Democrats likely have one Republican who will vote with them to call witnesses before the trial's end.

Republican Sen. John Cornyn told reporters on Monday that he thinks the Senate trial will begin ‪Jan. 21, first with opening arguments from both sides.

Then both sides will likely have an opportunity to call witnesses -- for the full chamber to vote on -- but only after opening arguments are wrapped up and senators have had a chance ask questions.

Though the president now appears to be supporting an outright dismissal of the articles, Senate Republicans are signaling that they have no intention of going this route.

"I think I'm safe in saying there's almost no interest in motion to dismiss and certainly there aren't 51 votes for a motion to dismiss," Sen. Roy Blunt, R-Mo., told reporters Monday night.

A new national Quinnipiac poll of registered voters found that a majority of Americans think former White House national security adviser John Bolton should be called to testify: 66% of registered voters, including 39% of Republicans and 71% of independents. Bolton has said he would come, if called.

Tracking with voters, Sen. Mitt Romney said Monday that he would like to hear from Bolton. Out there, publicly in favor first, could go a long way toward convincing other Republicans to join him in a call for witnesses too. Still, Democrats would need four of Republicans to vote with their full caucus for the motion on witnesses to carry.

The TIP with Cheyenne Haslett and Sasha Pezenik

Attempts by Warren, Sanders and some of their supporters to de-escalate their conflict on Monday night underlined a crucial irony: in the all-important conversation on how to beat Trump -- and who is best-suited to do it -- the fracture now splitting down the center of the progressive wing is being driven open by that very issue.

Beating Trump is the very thing Democrats all say they want and need to stay united on -- at the very moment it's dividing them most.

In the wake of the news of the Sanders-Warren dispute, supporters seemed to be trying to control the ripple effects and refocus. PCCC, a progressive PAC backing Warren, called the conversation "counterproductive."

"In this pivotal moment of the campaign, progressives must work together to defeat Donald Trump and prevent a less-electable establishment candidate like Joe Biden from getting the nomination," co-founders Stephanie Taylor and Adam Green said in a statement Monday evening.

THE PLAYLIST

ABC News' "Start Here" Podcast. Tuesday morning's episode features ABC News' Adam Kelsey, who joins us from Iowa to discuss new tensions between Sens. Bernie Sanders and Elizabeth Warren ahead of the last debate before the Iowa caucuses. Then, ABC News Chief Justice correspondent Pierre Thomas explains why Saudi military trainees are being kicked out of the program in the wake of last month's naval base shooting in Pensacola, Florida. http://apple.co/2HPocUL

FiveThirtyEight Politics Podcast. As the nation prepares for its third impeachment trial in history, "The Investigation" co-hosts Chris Vlasto and Katherine Faulders bring all the latest updates from Capitol Hill with ABC News senior congressional correspondent Mary Bruce. Then, ABC News senior national correspondent Terry Moran joins to set the historical context and significance of this impeachment proceeding, explaining that "this is the biggest thing that Congress can do, overturn an election, remove a president from office." https://apple.co/23r5y7w

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