On April 27, 2004, Senator Arlen Specter defeated then-Congressman Pat Toomey in Pennsylvania's Republican U.S. Senate primary. Specter went on to win a fifth term in the Senate, but that primary result was a close call. Specter won 50.8% to Toomey's 49.2%.
Toomey had been gearing up to again oppose Specter in the 2010 Republican primary. But the picture is now radically changed with Specter's switch to the Democratic Party. Now, it's quite possible that there will be such a rematch, but in the general election.
Toomey told local TV news here that he welcomes such a rematch. But I suspect he is, to quote a Hall and Oates lyric, wearing a mask of false bravado. It seems clear that it would have been much easier for Toomey to defeat Specter in the Republican primary, than in the November election. Toomey would then have faced some lesser-known Democratic name in the general election.
The general election is still a year and a half away, so too bold a prediction is perhaps not in order. But, unless there is a major trend back toward the Republicans in next year's election (such as the one in 1980 when Specter first won the Senate seat), I don't see how the stars can be aligned to produce a successful general election challenge to Specter from the right.