Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

WASHINGTON — Impeachment looms over this year's elections, whether either party's leadership wants that or not.

Even before "Fire and Fury" raised questions about President Donald Trump's mental fitness, 70 percent of Democratic voters told NBC News/Wall Street Journal pollsters they wanted congressional hearings on impeachment, making it impossible for their party to ignore the issue as they try to ride an anti-Trump wave to control of Congress.

Winning a majority in the House would theoretically give Democrats the power to impeach the president — which some liberal lawmakers have already predicted will happen — while it would take two-thirds of the Senate to vote convict and remove Trump from office.

But Democratic leaders in the House and Senate say impeachment is the last thing they want to talk about in this election cycle, and many party insiders feel campaigning on the issue could backfire.

Competitive Democratic primaries, pressure from the party's base, and millions of dollars from their single largest donor will make it impossible to ignore even if Special Counsel Robert Mueller drops no bombshells before November.

"Dancing around just to avoid using the 'i-word' will only convince people that politicians want to avoid one of the key questions at hand," said Ben Wikler, the Washington director of the liberal group MoveOn.org, which backed impeachment last June after Trump fired former FBI James Comey.

No one is calling for Democrats to make 2018 a referendum on impeachment, but California billionaire Tom Steyer — who spent nearly $90 million in the 2016 election, making him Democrats' biggest benefactor — wants it to be included in the mix.

Steyer has become the nation's prominent cheerleader for impeachment, and announced on Monday that he will spend $30 million on trying to flip the House this year and doubled down his support for a separate public relations campaign on impeachment that has already cost him $20 million.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Billionaire environmental activist Tom Steyer speaks during a news conference on Jan. 8, 2018 in Washington. Carolyn Kaster / AP

"We are not going to have a litmus test about impeachment," Steyer said. "What we are going to try to do is talk directly to the American people, so that people running for office know for a fact that the American people are behind impeachment."

While Steyer's two efforts are technically separate, the "Need to Impeach" campaign will use its massive email list to alert supporters if their Democratic representatives haven't weighed in on impeachment.

"We know this makes some of our friends and allies in this city uncomfortable," said Steyer, who spoke with House Minority Leader Nancy Pelosi, D-Calif., and other Democratic leaders before his press conference. "Millions of Americans know this president is unfit to be president. They need to know that a Democratic Congress would do what is morally and urgently necessary."

Neither party's leadership wants to turn Trump's first midterm election into a referendum on his removal.

"It's about the well-being of American families," House Minority Leader Nancy Pelosi, D-Calif., told a gathering organized by the Hearst publishers late last year. "He's just not that important. Impeachment would further divide us, not bring us together."

It's a sentiment she and other Democratic officials have repeated often in public, including when House Democrats joined with Republicans to overwhelmingly reject an impeachment resolution offered by one liberal lawmaker late last year.

"I don’t think that impeachment is something we should be talking about," Sen. Joe Manchin, D-W.Va., who is up for reelection this year in a state Trump won by 40 percentage points, said on NBC's "Meet the Press" in December.

The downside of Democrats' candidate boom this year is a slew of crowded primaries in key swing districts. And while impeachment may be good politics in a primary, it may be a liability in a general election, since Republicans already plan to paint their opponents as obstructionists more interested in opposing the president than governing.

"There are so many instances where these candidates are cannibalizing each other to get to the left-most position," Alex Smith, the executive director of America Rising, a Republican opposition research and rapid response group. "There's going to be a lot of money spent in these primaries and the candidates are going to get to positions on extreme left that they're going to have to answer for in the general (election)."

Still, Republicans eyeing Trump's middling approval ratings may not be spoiling for this particular fight.

"We always encourage our candidates to focus on the issues that their constituents actually care about," said National Republican Congressional Committee spokesperson Jesse Hunt. "More money in people's paychecks as a result of tax reform and Democrats' universal opposition to said reform is a perfect example."

Pelosi and other leaders successfully tamped down talk of impeaching George W. Bush as Democrats geared up for a wave in the 2006 midterms. Even after they won the House, Pelosi declared the issue "off the table."

Impeachment has already gained more steam under Trump than it did under Bush. And campaign finance changes have empowered donors like Steyer to drive issues even more so than party leaders are sometimes able.

And behind the scenes on Capitol Hill, the elevation of Rep. Jerrold Nadler, D-N.Y., to Democrats' top spot on the House Judiciary Committee was widely seen as laying the groundwork for a potential future impeachment push, given that he's a constitutional law expert.

Zephyr Teachout, a progressive activist and former congressional candidate, predicted that most candidates in competitive Democratic primaries will at least call for an impeachment investigation.

"This is a moral question, not a political one," she said.