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WASHINGTON — One of the biggest fights of this year's presidential election may be over the election itself and how to hold it during the coronavirus crisis.

Vote-by-mail, long mostly a hobbyhorse of good-government advocates, has suddenly been thrust to the center of an escalating partisan war as some say it's the only solution staging the November election if the pandemic continues.

And the mess in Wisconsin — which is holding in-person voting Tuesday after a partisan legal fight — shows that changing the election will not be easy, since the battle has become more about power than the already considerable logistical and legal issues.

President Donald Trump and other Republicans warn that a massive expansion of vote-by-mail could lead to their demise, while Democrats say failure to do so will disenfranchise millions.

Until now, the main factor as to whether a state embraced vote-by-mail was not its partisan lean, but its geographical location. West of Colorado, 69 percent of ballots are cast by mail, compared to only 27 percent of ballots nationwide, according to the National Vote at Home Institute.

Deeply conservative Utah has moved almost entirely to vote-by-mail in recent years while the Republican secretary of state in Washington is one of its biggest champions. Meanwhile, true-blue states like New York and several in New England have some of the more restrictive absentee balloting rules in the country.

But in the past few weeks, as the extent of the coronavirus outbreak became clearer and Democrats ramped up calls for national vote-by-mail, the partisan lines have sharpened.

Liberal-aligned groups that don't primarily focus on voting rights, such as the Sierra Club and abortion rights advocates, have joined the calls for vote-by-mail while conservatives have repurposed longstanding arguments about voter fraud to line up in opposition.

The tension sets up a battle in Congress over the next coronavirus relief measure, known to many on Capitol Hill as "Phase 4." Democratic leaders face growing pressure from their base — liberal activists and mainstream figures alike — to use the next aid package as leverage to ensure access to the ballot box by requiring every state to offer the option of voting by mail.

"With the insanity of Wisconsin, Democrats have the proof they need to make this a mandate for November," said Neera Tanden, the president of Center For American Progress, a think tank influential in Democratic circles.

She urged Democrats to do whatever they can to ensure vote-by-mail becomes law everywhere as a "fallback" in case the virus limits people from voting in person.

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Rep. Mark Pocan, D-Wis., the co-chair of the Congressional Progressive Caucus, wrote a letter to Speaker Nancy Pelosi, D-Calif., urging her to make it a priority to "enact a national vote-by-mail requirement for every federal election in 2020," while moderate Sen. Amy Klobuchar, D-Minn., has made vote-by-mail a focus since ending her presidential run.

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Pelosi included national vote-by-mail in Democrats' opening bid on the previous legislation, but it was removed due to Republican opposition. GOP aides cited security concerns and objected to using a coronavirus aid bill to overhaul election laws.

"It's a nonstarter," a senior Senate Republican staffer, who wasn't authorized to speak on the record, said of national vote-by-mail. "Republicans believe in federalism."

GOP opposition has only hardened since the last relief bill was passed two weeks ago, with Trump and his allies suggesting national vote-by-mail is little more than a Democratic plot to steal the election.

"You'd never have a Republican elected in this country again," Trump said last week on Fox News, referring to vote by mail.

During his daily briefing on the coronavirus Tuesday, Trump went even further. "Mail ballots are a very dangerous thing for this country because they’re cheaters," Trump said. "The mail ballots are corrupt, in my opinion."

That view did not stop Trump himself from requesting a mail-in ballot last month so he could vote absentee in Florida, where has now designated his Palm Beach club as his permanent residence. Trump also voted absentee in the 2018 midterms, the White House said at the time.

Trump also claimed in that briefing that Democrats did not try to move the Wisconsin election until he endorsed in the state Supreme Court race, but he actually endorsed in mid-January, nearly three months before the governor called off in-person voting.

"House Speaker Nancy Pelosi and former Vice President Joe Biden say we must throw election integrity to the wayside in favor of an all-mail election," Republican National Committee Chairwoman Ronna McDaniel wrote in a Fox News op-ed. "The overhaul would vastly expand opportunities for fraud and weaken confidence in our elections, but all Washington Democrats see is a potential benefit for their party."

Conservatives argue that removing the voting process from the watchful eyes of poll workers and other voters creates opportunities for mischief. But allegations of mail-ballot fraud have been exceedingly rare, with the most recent prominent example coming on behalf of a Republican congressional candidate in North Carolina in 2018.

Joe Biden, the leading Democratic presidential candidate, anticipated challenges to the election in an interview on NBC's "TODAY" show Tuesday, but said it must take place on Nov. 3 as scheduled and floated voting by mail as something "all the experts" say should be considered.

New legislation being crafted by Sen. Kamala Harris, D-Calif., would give states resources to carry out voting by mail. It is expected to include $1 billion to improve safety and efficiency of voting places, as well as encourage ways to cut lines to extend access to voting while social distancing, potentially with time slot reservations and curbside voting from one’s car, according to an aide familiar with the emerging bill.

Her fellow former presidential candidate, Sen. Elizabeth Warren, D-Mass., wrote Tuesday that Congress must use the next coronavirus relief to enact a number of voting provisions, including providing a vote-by-mail option.

The eight states that have switched to or are in the process of switching to entirely vote-by-mail say it is not only possible to protect the integrity of the ballot, but that they've been doing it successfully for years.

Kim Wyman, the Republican secretary of state of Washington, touted the fact that Washington now has the highest turnout of any state in the nation because everyone gets sent a ballot, and hoped educating critics about the security features would turn them around.

"The biggest challenge that states are going to face are the critics of vote-by-mail and people who don't understand it," she said.

But on the left, few think Republicans are being genuine in their concerns, arguing the real reason they want to stop vote-by-mail is precisely because states like Washington have such high turnout.

"These baseless attacks on vote-by-mail are a pathetic attempt to suppress the vote in the middle of a national crisis," said Ryan Thomas, spokesperson for the liberal group StandUp America.

Election experts say it's not clear that vote-by-mail would actually help Democrats, noting that plenty of Republicans still get elected in Utah and that Republican-leaning counties in Wisconsin have been returning absentee ballots at higher rates than Democratic-leaning ones.

Wisconsin is not the only battleground state where partisan fighting has already led to confusion about an election during the coronavirus crisis, even though it's only about a month old.

In North Carolina, the Republican leader of the state Senate rejected proposals by the state's top election official to make voting-by-mail easier, suggesting "progressive, liberal Democratic groups" wanted to make voter fraud easier, citing a recent instance of absentee ballot fraud on behalf of the GOP House candidate.

And in Georgia, the GOP secretary of state last month announced plans to send an absentee ballot to every active voter in the state's upcoming May 19 primary, but he's faced opposition from fellow Georgia Republicans.

"Vote by mail in my view is not acceptable," Georgia House Speaker David Ralston, a Republican, said in an interview with a local news site. "This will be extremely devastating to Republicans and conservatives in Georgia...This will certainly drive up turnout."

CORRECTION: (April 10, 2020, 1:17 p.m. ET): An earlier version of this article misstated the name of Washington's secretary of state. She is Kim Wyman, not Kay.