It's somewhat rare for incumbent Senators to face credible primary challenges, and rarer still for them to face challenges from a sitting member of Congress, as Joe Sestak is considering doing against Arlen Specter and Carolyn Maloney is considering doing against Kirsten Gillibrand. However, these challenges are not entirely without precedent. Since 1994, a sitting member of the House of Representatives has challenged an incumbent Senator from his own party on four occasions; two of the four challenges were successful. Although this is a very limited sample size, this is a better record than sitting members of the House have compiled in challenging an incumbent from the other party; such cross-party challenges have been successful (meaning that the challenger won both the primary and general election) on just 7 of 25 (28 percent) occasions since 1994.
The four intraparty challenges were as follows:
In 1996, Sam Brownback, then the representative from Kansas' 2nd Congressional District, challenged Shelia Frahm, who had been appointed to the Senate by governor Bill Graves to fill the seat vacated by Bob Dole (who had quit the Senate to demonstrate his commitment to his Presidential campaign). Brownback defeated Frahm in the primary and won the general election.
In 2002, NH-1's John E. Sununu defeated incumbent Robert Smith to win the Republican primary, before going on to defeat Jeanne Shaheen in the general election. (Shaheen, as you know, would defeat Sununu in a rematch six years later). Smith, however, was a somewhat unorthodox Republican, having briefly left the Republican party to seek the presidential nomination of the U.S. Tax Party (now the Constitution Party) and then becoming an independent before returning to the Republican caucus.
In 2004, conservative U.S. Rep. Pat Toomey, then of Pennsylvania's 15th Congressional District, challenged Arlen Specter for the Republican nomination and nearly defeated him, losing by only 17,146 votes. Toomey's desire for a rematch, of course, is what prompted Arlen Specter's defection from the Republican Party.
Lastly, in 2006, Ed Case, the Representative from Hawaii's 2nd Congressional District, challenged longtime Democratic incumbent Daniel Akaka. Akaka defeated Case 54-45 and went on, as expected, to win the general election, beating Republican Cynthia Thielen.
The motivation for the challenge was different in each instance. Case, although clearly to Akaka's right, was running principally on generational themes, arguing that the 82-year old Akaka was a liability to Democrats if he were to die or retire from office, with his replacement to be named by Republican governor Linda Lingle. Frahm, being an appointed Senator rather than a true incumbent, was inherently somewhat vulnerable. But there was little ideological daylight between she and Brownback; Frahm's DW-NOMINATE score, in her brief tenure in the Senate, was a rather conservative +.523, and Brownback's in his first term was a nearly identical +.503.
The case of Sununu and Bob Smith bears some parallels with the situation that Arlen Specter now finds himself in. Smith had an orthodox but generally very conservative voting record. But Republicans, as the New York Times reported, were still furious with Smith for having temporarily left the party to run for President as an independent and were glad do see him go; many sitting Republican Senators in fact endorsed Sununu.
Finally, there is the case of Toomey, who was far to Specter's right and challenged him almost purely along ideological grounds.
If there's a lesson here, it's that such challenges are not undertaken frivolously. Brownback and Sununu, certainly, had good reason to think they would win, challenging a pseudo-incumbent and an incumbent who had made too many enemies within his own party, respectively. Toomey somewhat self-evidently had good reason to think his challenge might succeed. Only Case's instincts, in retrospect, were a little bit questionable, although even he came within single digits.
Sestak and Maloney are also likely to undertake their challenges only if they determine they have a good chance of success. The situation is so dynamic in Pennsylvania that it is probably too early to make an honest assessment of Sestak's prospects, but Maloney is already within 5 points of Gillibrand in a hypothetical matchup.