Story highlights Sen. Cory Gardner said in a statement that Moore should be expelled from the Senate if he's elected

Only 15 members have ever been expelled from the US Senate, most during the Civil War

Sen. Lisa Murkowski most recently won a Senate write-in campaign in Alaska in 2010

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.

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