The 2016 presidential race has long been unusual, but now we are in completely uncharted territory. Donald Trump is at risk of becoming the first major presidential candidate in the modern era repudiated by his political party, a month away from the general election and hours before the second debate.

William Howard Taft suffered mass defections when his predecessor bolted the Republican Party to run for president on the Bull Moose ticket. Many high-profile Republicans endorsed against Barry Goldwater in 1964 or otherwise refused to support him.

Sitting Republican senators are calling for Trump to actually be replaced as the party's presidential nominee in response to a leaked recording of his lewd sexual comments about a woman. Some party leaders are said to be actively exploring their options for replacing Trump even at this late date.

Trump's remarks were condemned by the Republican speaker of the House, the Republican National Committee chairman who has been criticized by anti-Trump conservatives for his steadfast support for the nominee, vice presidential nominee Mike Pence and even his own wife, Melania Trump.

Perhaps most stunning, Trump's fall from grace comes less than a month after he pulled even with or ahead of Hillary Clinton in national and battleground state polls. While Trump has since slipped after a widely panned first debate performance, the latest GOP rebellion against the nominee comes without any public polls measuring the full extent of the popular backlash against him — or whether Republicans are vulnerable to an anti-anti-Trump backlash.

On paper, there isn't much the Republican Party or anyone else can do. Even if you accept the most expansive interpretation of the Republican National's Committee's Rule 9 for filling vacancies in nominations — and any reading that suggests it allows for the involuntary replacement of duly nominated candidates is highly debatable at best — there are a series of practical obstacles to dumping Trump.

Early voting has already started in some states. Deadlines for removing Trump for the ballot have passed in states that are critical to any realistic chance the GOP has for success in the Electoral College. Many ballots have already been printed. Trump seems all but certain to fight any serious effort to boot him in court, potentially keeping the legal matters from being resolved until after Election Day.

It's possible the RNC could encourage a revolt by faithless Republican electors, perhaps encouraging votes for Pence rather than Trump at the top of the ticket, and then fight state laws against such practices in court. But there is no guarantee that would prevail either in court or at reaching the necessary 270 to be elected president.

Yet we are in such an unprecedented political situation, it does remain an open question what the courts would actually allow. Judges have sometimes ignored what looked like the clear language of election statutes in the interest of promoting democratic competition. Might they do so again if the alternative was an effectively one-party presidential race?

Democrats were allowed to replace scandal-tainted, free-falling Sen. Robert Torricelli with former Sen. Frank Lautenberg on the New Jersey ballot. Republicans were not permitted to do the same thing with Rep. Mark Foley in Florida after his sexually explicit messages to pages.

Trump could step aside as GOP presidential nominee or promise to resign in favor of Pence if elected. But does either scenario seem terribly likely given what we know of Trump?

Would even an informal understanding that Pence was likely to become president work? When Democratic Senate Mel Carnahan died in 2000 before Election Day but after he could be replaced on the ballot, he won with the understanding that his wife would be appointed to the seat. But even former Vice President Walter Mondale couldn't hold the late Sen. Paul Wellstone's Senate seat for the Democrats under a similar set of circumstances two years later.

Most of the Republicans who have openly called for Trump's removal fit into at least one, and often two, of three broad categories: already anti-Trump; lukewarm Trump supporters; politicians who are themselves most vulnerable to an anti-Trump backlash. The candidate will perhaps need more loyalists to abandon ship in order to decide to end the voyage himself.

But the truth is, we've never seen anything quite like this. What will happen next is anyone's guess and may be something totally unprecedented itself.