The outrage ginned up over Donald Trump's meeting with President Putin has re-invigorated the Democratic party base and is turning the issue of impeachment into both a rallying cry and a litmus test for Democratic candidates around the country.

Nothing gets the liberal crazies more energized than the prospect of impeaching Trump.  This worries the national Democratic leadership because its members know that nothing would be more detrimental to their chances of retaking Congress than not only appearing to be off their nuts by calling for the impeachment of a president with no evidence that he's done anything wrong, but also energizing the Republican base, motivating Republicans to come out and vote.

But the leadership may not have much of a choice.  With pressure coming from below, even prominent Democrats are entertaining the idea.

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But where abolishing ICE fizzled for now as a progressive cause, the impeachment controversy could have sturdier legs.  At the least, it has forced Democratic leaders to confront and carefully manage pressure from the base. 

House Minority Whip Steny Hoyer, D-Md., on Tuesday charged that Trump's remarks in Helsinki, Finland, were "nothing short of treason" and marked a "dark day" in American history. 

Hoyer characterized impeachment, though, as a "distraction," while still keeping the door open for the process in the future.  "There will be time enough to deal with other issues and oversight of the president's performance when we take back control of the House of Representatives," he said. 

Rep. Eric Swalwell, D-Calif., a member of the House Judiciary Committee, toed the same line on Monday.

"I think that's premature at this point – we should do all we can to make sure that he's held accountable, that we conduct the investigations the Republicans have been unwilling to do," Swalwell said in an interview with The Hill.

"If impeachment is the case," he added, "it's because we found impenetrable evidence that we take to the American people and will be accepted by both Republicans and Democrats."

"Impenetrable" evidence will almost certainly not be forthcoming.  But the rabid base doesn't care about proving anything.  They want Trump gone by any means necessary.

Obama-era CIA Director John Brennan all but endorsed impeachment when he tweeted that "Donald Trump's press conference performance in Helsinki rises to & exceeds the threshold of 'high crimes & misdemeanors'" and was "nothing short of treasonous."

Ron Fein, with Free Speech for People, wrote in The Washington Post that while Trump's conduct might not rise to the level of treason, "it might still be impeachment-worthy regardless."

In perhaps the most dramatic sign that the impeachment fight may go mainstream, Texas Rep. Beto O'Rourke – he is running to unseat GOP Sen. Ted Cruz in November in the deep-red, pro-Trump state – has both advocated impeaching Trump and downplayed the Democratic consequences of such an effort.

"Impeachment, much like an indictment, shows that there is enough there for the case to proceed," O'Rourke said this week, "and at this point there is certainly enough there for the case to proceed."

Democrats play with this issue at their own risk.  Impeachment is being driven by hysterical, exaggerated, unsubstantiated media stories largely based on anonymous sources or selective, illegal leaks from the Mueller investigation.  It is a media-driven narrative augmented by wild speculation by liberal pundits and commentators that Trump is a traitor, that Putin "has something" on him, that the president is repaying Putin for getting him elected, that Trump is actually conspiring with Putin to advance Russian interests.

None of this is remotely provable, but that hasn't lessened the frenzy.  If Democrats choose to make impeachment a litmus test, they will be met with a mostly united GOP, energized and determined.  That is exactly the opposite of what they need to retake the House: a divided, dispirited Republican Party where activists stay home on election day.