The fallout over leaked audio of Donald Trump using lewd language about women has some Republicans calling on him to withdraw from the presidential race. Trump told the Wall Street Journal Saturday there is "zero chance" he will withdraw. And any such move would be legally difficult because voting has already begun across the nation.
Under the Republican Party's rules, party leaders can pick a replacement if a nominee withdraws.
The Republican National Committee, a group of 168 party leaders and activists from across the country, is empowered to name a replacement in the event of a vacancy for "death, declination, or otherwise," under Rule 9.
If Trump withdrew, the RNC could tap vice presidential nominee Mike Pence, or any other eligible citizen, to be the party's nominee.
That step, however, would only cover the party's role. Whether voters could actually pick the new nominee would depend on 50 different sets of state laws.
At least 34 states offer early voting, and several only allow voting by mail, which means Trump ballots have been printed and sent out. In fact, about 20 percent of the electorate already has the option of early voting this week, according to data from the U.S. Election Assistance Commission analyzed by Roll Call magazine.
Beyond the practical matter that people are already voting for and against Trump right now, the registration deadlines for most states have long passed. That means most states have no legal obligation to change their ballots for any new nominee.
Elections are highly regulated events, however, and courts have ordered states to reasonably extend or revise deadlines for extraordinary circumstances.
Most precedent on the issue relates to the death of a candidate. In 2000, for example, a Missouri law prevented a new candidate being placed on the ballot to replace Mel Carnahan, who died weeks before the election. He still won the most votes, however, and the state's governor honored his pre-election pledge to appoint Carnahan's widow as the new senator.
Other states have more ominous precedent for candidates who emerge after ballots are printed. Minnesota courts partially restricted how new ballots could be provided to voters in the wake Sen. Paul Wellstone's death late in the 2002 campaign.
To advance a new presidential nominee at this point, the GOP would have to file suit in every state that declined to voluntarily change its ballots, asking the courts to force the new nominee's name onto the ballot.
That presents terrible legal odds - even if the party won 80 percent of those challenges, it could face disqualification or confusion in ten other states, a death knell for a close presidential race.
A party pushing a candidate who is not on the ballot has one final hail marry under the U.S. Constitution - resorting to intervention by electors in the Electoral College.
Technically, the President is still chosen by the Electoral College. Most states have laws binding their electors to back the winner of their state's popular vote, but others do not.
If a state did not put the new candidate on the ballot, and a party's previous candidate still won the popular vote, the party could conceivably argue that the electors should back the party's new candidate. If those states happened to control the outcome of the election, the issue would almost certainly litigated to the Supreme Court.
The court is currently split 4-4 on many key issues, since the Senate has declined to act on President Obama's nominee.
The entire notion of such a legal-political standoff seems fanciful and completely unlikely, because it would require so many unusual and unprecedented events transpiring in the final weeks of the election.