US House Speaker Nancy Pelosi (2ndL), flanked by (fromL) House Intelligence Committee Chairman Adam Schiff, US Representative Elijah Cummings, Democrat of Maryland and Chairman of the House Oversight and Reform Committee and Chairman of the House Judiciary Committee US Representative Jerry Nadler, delivers a press conference following the former Special Counsel's testimony before the House Select Committee on Intelligence in Washington, DC, on July 24, 2019. (Photo by ANDREW CABALLERO-REYNOLDS / AFP) (Photo credit should read ANDREW CABALLERO-REYNOLDS/AFP/Getty Images)

If House Democrats want President Trump held formally accountable for his conduct surrounding Russian election interference, they'll need to do it all on their own.

Robert Mueller won't do it. As the former Special Counsel reminded lawmakers on Wednesday, the Justice Department says the Constitution shields sitting presidents from prosecution.

Rank-and-file Americans won't demand it. Polling shows the public split on whether Congress should impeach Trump, and hours of Mueller's testimony offered no dramatic revelations with obvious potential to change that.

House Republicans certainly won't help. GOP lawmakers, who protected Trump from scrutiny when in the majority, focused in Judiciary and Intelligence committee hearings on ripping the investigation rather than exploring what it found.

That leaves Democrats alone with their consciences, their constituents, and their cost-benefit analyses of how impeachment would affect the party's ability to retain its majority and stop Trump from winning re-election in November 2020. Their diverse 235-member caucus faces tough choices.

Speaker Nancy Pelosi insists Democrats won't impeach or duck impeachment on the basis of political consequences. So far, in resisting the impeachment demands of colleagues, she has given every impression of doing exactly that.

History and current circumstances explain her reluctance.

Two decades ago, Pelosi watched a Republican majority stumble through its impeachment of Bill Clinton and then oust Speaker Newt Gingrich. Pelosi's majority includes 31 members who represent districts Trump carried in 2016 and could face electoral danger. Impeachment might accomplish little more than energizing Trump 2020 voters.

Close Pelosi allies insist she couldn't gain majority support for impeachment even if she tried, not to mention the two-thirds of a Republican-run Senate needed for conviction and removal from office. "There will never be 218 in the House," a leadership aide told me.

The damning results of the special counsel investigation weigh against that conclusion - however tersely and haltingly Mueller discussed them before television cameras.

Before concluding his work, Mueller obtained felony convictions against Trump's campaign chairman, deputy chairman, National Security Adviser and personal lawyer.