Republicans working on the Senate race in Colorado sensed something was shifting even before the votes were counted.

Voters in suburban Denver — who had backed President Obama twice — were abandoning Senator Mark Udall, a Democrat, in the final days before the election.

“We thought the evening could be decided early,” said Cory Gardner, the Republican who defeated Mr. Udall.

More striking than any Republican gains in red-state America on Tuesday were the party’s Senate victories in Colorado and North Carolina and the near miss in Virginia.