Allegations that Roy S. Moore, the Republican nominee for a Senate seat in Alabama, pursued sexual and romantic relationships with teenagers while he was in his 30s have upended a race in a state that has not had a Democratic senator since 1997.

While Alabama Republicans, by and large, defended Mr. Moore against what many of them described as a partisan plot, national officials have reacted with shock and disgust. And the shift away from him has been particularly pronounced in the chamber he hopes to join.

On Monday, Senator Mitch McConnell, the Republican leader, said Mr. Moore “should step aside.” When asked by reporters whether he believed the allegations, he replied, “I believe the women, yes.”

The National Republican Senate Committee is no longer raising money for Mr. Moore, and most Republican senators say he should end his campaign if the allegations — reported by The Washington Post on Thursday and based on more than 30 sources, including four accusers quoted by name — are true. A fifth woman came forward on Monday.