'You can't impeach,' Trump says. Don't expect Democrats to try to prove him wrong.

At least two 2020 U.S. Democratic presidential candidates are explicitly calling for it. A third candidate says it shouldn't be taken off the table. And two liberal Democratic lawmakers chairing powerful House committees want it, despite warnings against it from Speaker of the House Nancy Pelosi.

Yet, as the impeachment drumbeat builds after U.S. Special Counsel Robert Mueller's report that detailed questionable conduct by Donald Trump and his 2016 presidential campaign, the notion also carries great risk for Democrats now grappling with whether the political process could hurt their election odds.

Impeachment, which would formally censure the president, is a step that could precede Trump's possible — but unlikely — removal from the Oval Office.

In a tweet on Monday, Trump wrote: "There were no crimes by me (No Collusion, No Obstruction), so you can't impeach."

Asked later by reporters whether he was concerned about the ramped-up impeachment talk, he responded, "Not even a little bit."

The question of whether or not to invoke impeachment proceedings presents a quandary for Democrats: Exercise their constitutional responsibilities of government oversight or risk angering swing voters wary of political overreach?

Released last Thursday, Mueller's 448-page redacted report into Russian interference in the 2016 election laid out at least 11 episodes of possible obstruction.

Even so, it found insufficient evidence to charge Trump or his campaign aides with criminally conspiring with Moscow to help Trump win, despite a litany of Russian outreach efforts and examples of the Trump campaign welcoming the help. Nor did the report result in charges of obstruction, due in part to the special counsel adhering to a Justice Department policy that says a sitting president can't be indicted.

Harris, Warren call for impeachment

But that doesn't stop Congress from acting on impeachment, the special counsel suggests. Indeed, experts on the political process say Mueller's report makes for a case for lawmakers initiating impeachment proceedings against the U.S. president for obstruction of justice.

On page 220 of the report, the Special Counsel's Office writes: "The conclusion that Congress may apply the obstruction laws to the president's corrupt exercise of the powers of office accords with our constitutional system of checks and balances and the principle that no person is above the law."

That, as former federal prosecutor Mark Osler saw it, amounts to "a soft invitation to impeachment."

The very idea likely raises anxieties among Democratic leaders like Pelosi. Last month, the House Speaker told the Washington Post, "I don't think we should go down that path because it divides the country. He's just not worth it."

Some Democrats beg to differ. On Monday night, Sen. Kamala Harris, who is running to be president in 2020, told a CNN Town Hall, "I believe Congress should take the steps towards impeachment." Days earlier, Sen. Elizabeth Warren tweeted on Friday afternoon that the "severity" of Trump's actions demands that Congress exercise its constitutional duties of oversight.

"That means the House should initiate the impeachment proceedings against the president of the United States," she tweeted.

The same day, 2020 Democratic presidential hopeful Julian Castro told CNN he considers it "perfectly reasonable" to call for impeachment proceedings after what Mueller found. And Democratic House members Alexandria Ocasio-Cortez, Rashida Tlaib and Ilhan Omar are now backing calls for an impeachment inquiry.

There appears to be enough damning content in Mueller's treatise to warrant those first steps, said Michael Gerhardt, a scholar on federal impeachment with the University of North Carolina at Chapel Hill.

He believes the special counsel last week suggested an impeachment process to Congress for Trump-related obstruction, based on nearly a dozen episodes outlined by Mueller, including an attempt to engineer the firing of the special counsel, an attempt to limit Mueller's investigative scope to only future election meddling and the successful firing of former FBI director James Comey.

"If anything has been strengthened, it's the case to initiate an inquiry or a hearing," Gerhardt said. That process would be a call for additional fact-finding and investigation. Historic precedent already supports obstruction of justice as grounds for impeachment going back to the Nixon administration.

With a majority in the House, Democrats have the power to impeach the president. But Trump is likely to remain safe from being convicted in the Republican-controlled Senate, the next step that would actually lead to his removal from office. Unless Democrats have the popular sentiment to back them up on impeachment, the effort itself might look like an undemocratic move to try to eject the president.

Those considerations will be weighing on the party in the year before an election, said Ross Garber, a leading Washington, D.C., impeachment lawyer who has represented four U.S. governors in impeachment proceedings.

Clinton impeachment serves as warning

"It's Congress's responsibility to now evaluate whether that's a direction that it really wants to go in," he said. "The challenging thing is it does potentially have a political boomerang effect in terms of motivating the president's base."

Older members of Congress lived through President Bill Clinton's impeachment on perjury and obstruction charges in 1998. Voters reacted against the Republicans, who were seen as overreaching, and Republicans lost big in the next election.

"After the Clinton impeachment effort, he was actually more popular," Garber said.

What's more, ousting Trump would require a two-thirds majority to convict him in the Senate, meaning at least 20 Republicans would have to side with every Democratic senator to convict. That's a slim prospect.

Recent polling suggests Americans aren't clamouring for impeachment, with the president maintaining steady favourability numbers around 40 per cent despite the report's release. A Monmouth University poll last week showing most Americans (54 per cent) want Congress to move on after Mueller's report.

On the one hand, Garber said, some people might view impeachment of Trump as "a core constitutional responsibility that shouldn't be governed by the polls."

Political historian Allan Lichtman, author of The Case for Impeachment, took an unwavering view.

"It is absolutely their constitutional duty to have an impeachment inquiry," he said. "It is not their constitutional duty to look into a crystal ball and try to figure what the Senate might or might not do after a trial."

Recalling Clinton's impeachment, Democratic House oversight committee chairman Elijah Cummings told MSNBC on Friday what Trump was found to have done was "at least 100 times worse."

In a tweet, he said he had been fielding queries about impeachment, "and we may very well come to that very soon."

House majority leader Steny Hoyer dismissed impeachment as "not worthwhile." He said Friday that he prefers ousting Trump from the White House in the 2020 election.

That would appear to be a safer political route, said Robert Deitz, a former general counsel for the National Security Agency.

"What concerns me is [Republicans] are going to say Democrats are so fixated on Trump they've lost their minds," he said of the political calculations. "It's a tough call."

But Deitz said that Congress has oversight responsibilities, too.

"And one is likely to conclude that if you don't nip Trump-esque behaviour in the bud, that's going to become the new normal."

The election is 19 months away, but it would let voters decide at the ballot box whether the president is fit to serve, said Garber, the impeachment lawyer.

"Both the impeachment process and elections are ways to define standards," he said. "Those are both referenda on the president's conduct, and the extent to which it's acceptable, both to members of Congress and the American people."