On Tuesday, Democratic voters in Connecticut will cast their ballots to consider denying renomination to Joe Lieberman. The three-term incumbent trailed in the final pre-primary poll by 13 points; two-thirds of those who supported his opponent, Ned Lamont, said they did so because of their opposition to Lieberman and his policies, rather than their positive feelings toward Lamont. Those are hardly encouraging figures for Lieberman, who has said he'll run as an independent if he comes second in the primary.

It is very rare for incumbent senators to lose in their party's primary: since 1960, only 19 have so fallen. Five of these were special cases: Sheila Frahm (R-Kansas), Donald Stewart (D-Alabama), David Gambrell (D-Georgia), Howard Metzenbaum (D-Ohio, in 1974), and Ross Bass (D-Tennessee) were either interim appointments or had won special elections, and then failed to secure the nomination for a full term in the Senate. Advanced age (J. William Fulbright, D-Arkansas; B. Everett Jordan, D-North Carolina) or scandal (Edward Long, D-Missouri; A. Willis Robertson, D-Virginia) explained the defeats of four others.

A pattern of political weakness explained the primary defeats of three other senators. In 2002, New Hampshire Republicans ousted Robert Smith in large part because they believed, probably correctly, that he couldn't win in November; he had barely won re-election in 1996, in a race that TV networks prematurely called for his opponent, former congressman Dick Swett. Two other spurned incumbents, Democrats Mike Gravel of Alaska and Richard (Dick) Stone of Florida, had initially captured very tight primaries and never established firm party bases. Stone lost to the man who he had defeated in 1974, former congressman Bill Gunter; Gravel fell to Clark Gruening, the grandson of the senator he had ousted in 1968. Both Gunter and Gruening then lost the fall election to Republicans.

Ideology and changing party politics played a decisive role in the failed renomination bids of five senators. Three defeated Republican moderates (Jacob Javits, R-New York; Clifford Case, R-New Jersey; Thomas Kuchel, R-California) were victims of the GOP's shift to the right: the party that ousted them was very different ideologically than the party that initially nominated them. The conservatives who prevailed, however, were all flawed candidates. In 1968, Max Rafferty lost to Democrat Alan Cranston—and Kuchel's seat has remained in Democratic hands ever since. In 1978, Jeffrey Bell lost to Bill Bradley—and Case's seat has remained under Democratic control ever since. In 1980, Jacob Javits lost to Al D'Amato—who surely would have been defeated in November but for Javits' continued presence on the ballot, as the Liberal Party nominee. The incumbent senator siphoned a critical 11 percent of the vote away from Democrat Liz Holtzman, allowing D'Amato to prevail by one percentage point, 44-43.

On the Democratic side, the effects of the Vietnam War and their states' more general shift to the right contributed to the primary defeats of Ernest Gruening (D-Alaska) in 1968 and Ralph Yarborough (D-Texas) in 1970. Gruening, seeking a third term at age 81, came under attack for his increasingly extreme antiwar position (Gruening had voted against all defense appropriations after 1965) and his disinterest in bringing home federal projects to Alaska. Gruening's foreign policy radicalism and New Deal liberalism was out of place in a state transforming from a Democratic bastion (at time of statehood, 90 percent of Alaska's state legislature were Democrats) to the libertarian frontier it is today. Age and ideology also hurt the last of the Texas liberals, Ralph Yarborough, who was out-spent by conservative Lloyd Bentsen in the 1970 Democratic primary. Bentsen enjoyed covert support from LBJ, who remembered Yarborough's opposition to his administration's policy in Vietnam; the challenger also put together a devastating ad linking Yarborough to the riots at the 1968 Democratic convention.

The closest parallels to the situation Lieberman has faced this year came in Illinois in 1992 and Ohio in 1968. In Illinois, incumbent Alan Dixon had a profile remarkably similar to Lieberman's. He was a genial, two-term incumbent, with a moderate voting record, someone known for his ability to work with senators across the aisle. Like Lieberman, he had enjoyed smashing electoral success (taking an open-seat race in the bad Democratic year of 1980, securing two-thirds of the vote in 1986) but was sometimes overshadowed by his in-state colleague—Dodd in the case of Lieberman; for Dixon, Democrat Paul Simon. Also like Lieberman, Dixon departed from his Democratic colleagues on an emotionally charged issue: he was the only northern Democrat to support Clarence Thomas's confirmation to the Supreme Court in 1991.

In light of the Thomas vote, the bipartisan, moderate image that once was Dixon's greatest asset instead became a major liability. His vote prompted an immediate primary challenge from the up-and-coming Cook County Recorder of Deeds, Carol Moseley-Braun. Sensing an opportunity, personal injury lawyer Al Hofeld jumped into the race, launching a slashing negative campaign that (not inaccurately) portrayed Dixon as an ineffective backbencher. Dixon countered by attacking Hofeld, only to see Moseley-Braun slip by both to take the primary with just under 40 percent. Hofeld and Moseley-Braun in many ways represented a combined version of Ned Lamont in 2006: Hofeld had Lamont's personal wealth, Moseley-Braun his appeal to grassroots liberals. The first African-American woman ever to win election to the Senate, Moseley-Braun served a term before being unseated by Peter Fitzgerald in 1998.

Unlike the Illinois three-way 1992 primary, two-term Democrat Frank Lausche was unseated in a two-way contest in 1968. Ohio's governor before winning election to the Senate in 1956, Lausche drifted to the right after easily capturing a second term in 1962. One of the most passionate defenders of LBJ's Vietnam policy in the Senate, he regularly called into question the patriotism of dissenters on the Foreign Relations Committee. (He frequently complained of the torture of having to serve on the same committee as Franck Church.)

Throughout his career, Lausche had enjoyed bipartisan appeal in a state that leaned Republican, but as 1968 approached, it was clear he was vulnerable in the primary—as polls showed him (much like Lieberman in 2006) more popular among his state's Republicans than among Democrats. Former congressman (and future governor) John Gilligan, promising to support liberal policies at home and oppose the war abroad, unseated him in a bitter primary—only to see many Lausche supporters defect to Republican nominee William Saxbe, who prevailed in the fall.

Much like Lausche and Dixon, Lieberman seems to have crossed a threshold, where what once appeared to be commendable moderate instincts came to be seen by many Democrats as partisan apostasy. Unlike Lausche and Dixon, however, the Connecticut senator has promised to fight on even if he loses the primary. If he chose to pursue this course, he would hope to become the first person since John Warner in Virginia (1978) to be elected to the Senate after losing in his party's nominating process.