They got there by people, their constituencies, voting for them. They got there by people voting the 'party line ticket,' instead of waiting/requiring a better candidate. The GOP is in the lousy position it is in not because people stayed home in protest of voting or voting third party. The GOP is in the position it is in because they convinced the electorate we needed to keep the seat, despite the candidate failing to meet basic core conservative standards.
That doesn't explain why Bill Sali (R-ID), one of the most conservative congressmen out there, lost his re-election bid two months ago.
It costs so much to win a Senate seat that all you can get is the Country Club branch of the party.