Washington (CNN) - The pressure just keeps on mounting on Alabama Republican Roy Moore.
The 70-year-old US Senate hopeful, now less than a month from a special election to replace Attorney General Jeff Sessions, has been accused of pursuing relationships with teenage girls while he was in his 30s. One woman said she was 14 years old when Moore initiated sexual contact with her. And on Monday, another Alabama woman alleged Moore sexually assaulted her when she was 16.
Senate Majority Leader Mitch McConnell and majority whip John Cornyn, along with several more rank-and-file senators, withdrew their endorsements of Moore on Monday.
There's just one problem for the GOP: It's too late to get Roy Moore off the ballot.
Senate president pro tempore Orrin Hatch tweeted on Monday that sitting Sen. Luther Strange, who was appointed in the immediate aftermath of Sessions' resignation, should keep the seat.
But CNN's Manu Raju reports top Senate Republicans are skeptical that Strange, who lost to Moore in a primary runoff, could mount a successful write-in campaign, according to two senior GOP sources.
And to make matters worse for Moore, the head of the Senate GOP's campaign arm, Sen. Cory Gardner, said in a statement that Moore should be expelled from the Senate if he's elected.
So what's next? If Moore bows out, Alabama Republicans will need to scramble to push another candidate across the finish line, aiming to become one of the few Senate candidates in modern US history to win a write-in campaign.
But if Moore doesn't step aside, the Senate may try to make him the first member since the Civil War to be expelled from the deliberative body.
Or perhaps the worst outcome for Republicans is an outright victory for Democratic candidate Doug Jones, shrinking the already-too-close-for-comfort 52-seat GOP majority to just 51 seats.
The rarity of Senate expulsion
It would take the votes of two-thirds of the members in the Senate to oust Moore from his seat -- and it's only happened 15 times in American history.
Fourteen of them were expelled for supporting the Confederacy during the Civil War -- 10 of them were ousted at once when they didn't show up in July 1861. The other expulsion came earlier, when Tennessee's William Blount backed a plan to overthrow Spain in Florida and give the territory to Great Britain in 1797, according to the Senate historian.
No members of the Senate have been booted from office since the Civil War, though expulsion proceedings have been initiated at least a dozen times since then for everything from election fraud to embezzlement to Mormonism, according to the Senate Historical Office.
Most recently, in 1995, Oregon Sen. Robert Packwood was recommended for expulsion by a Senate panel for sexual misconduct and abuse of power. He resigned less than a month later.
The longshot path: A write-in campaign
Only twice in American history has a write-in candidate won a general election battle.
Most recently, in 2010, Alaska Sen. Lisa Murkowski lost her primary election to Joe Miller. And despite the urging of national Republicans, the incumbent jumped into the race as a write-in candidate.
Former Gov. Sarah Palin called her bid a "futile effort" at the time, and the National Republican Senatorial Committee urged voters -- and Murkowski -- to fall in line behind Miller. Murkowski won the race with 39% of the vote vs. 35% for Miller and 23% for the Democrat.
Before that, in 1954, Strom Thurmond launched an independent bid in South Carolina after the state party opted not to hold an open primary after sitting Sen. Burnet Maybank died. Thurmond won with 63% of the vote and sat in the Senate for nearly a half century.
(William Knowland from California won a write-in campaign in late 1946 for a two-month special election while he was also listed on the ballot for the full Senate term beginning in January 1947. Glen Taylor in Idaho and Ernest Gruening in Alaska launched unsuccessful write-in bids in 1956 and 1968, according to the Senate Historical Office.)
On the House side, at least four candidates have won write-in campaigns in modern US history, according to the Biographical Directory of the US Congress: Charles Forrest Curry Jr. from California in 1930, Thomas Dale Alford in Arkansas in 1958, Joseph Skeen in 1980 and Ron Packard in 1982.