Lyndon Johnson used to tell a joke: “What is the difference between liberals and cannibals? Cannibals don’t eat their friends and family members.”

The polarization of political elites is increasing the drumbeat intra-party cannibalism. With the rigged system of redistricting creating a 98% congressional re-election rates, and activist bloggers calling for the electoral execution of any official they deem to be insufficiently ideological, party primaries are fast becoming prime electoral fights.

With control of Congress possibly hanging in the balance, and both parties struggling to find their souls in advance of 2008, the impact of this catching cannibalism could be considerable. It’s also selfish and stupid.

When a Democratic senator consistently gets 60% of the popular vote in a state with a moderate Republican heritage and Republican governors for the past 12 years, he should be considered a powerful asset. But if you’re one of the increasingly angry left-wing Democrats, you demonize him and then proceed with a primary.

That’s the position that a Connecticut senator and former vice presidential candidate, Joe Lieberman, finds himself in this year. Despite his more than 80% rating from Americans for Democratic Action and a low 20% rating from the American Conservative Union, he is being derided as a DINO – a Democrat In Name Only – by left-wing advocates and some party influencers, such as Howard Dean’s brother.

Amid President Bush’s increasing unpopularity in the state in which he was born (Mr. Bush was born in New Haven, Mr. Lieberman’s hometown), Mr. Lieberman’s support for the war on terrorism has made him persona non grata among left-wing Democrats. They have found their champion in a political neophyte and local businessman, Ned Lamont. One self identified Lamont delegate on the Daily Kos blog expressed the liberal rational for Mr. Lieberman’s replacement: “For years he has disparaged fellow Democrats and, time and again, provided a bi-partisan shield for Republicans’ harsh right-wing policies.” In their partisan fervor, such Democrats are reviving the “love it or leave it” rationale they criticized conservatives for using in the past, but now they are applying it to their party instead of patriotism.

Faced with this challenge, Mr. Lieberman’s recent statement that he would not rule out an independent candidacy if he loses the August primary set off waves of speculation. While it was an offhand comment, it underscores the fact that Mr. Lieberman could easily win as an independent. The two parties’ primaries are increasingly dominated by their more extremist voices, a situation unhealthy to the interests of a genuinely representative democracy.

Slightly northeast, in Rhode Island, a Republican senator, Lincoln Chafee, finds himself in a similar position. Identified as one of the most vulnerable senatorial incumbents in a state that has been deeply Democratic for years, Mr. Chafee – son of the popular former senator – was, nonetheless, able to win election in 2000 by commanding 57% of the popular vote. To put Mr. Chafee’s senatorial record in context, he has a 53% rating from the American Conservative Union and a 45% rating from the Americans for Democratic Action. That is a balanced record that reflects both his conscience and his constituents.

If Mr. Chafee were to lose his party’s primary to the mayor of Cranston, R.I., Stephen P. Laffey – who is being backed by the Club for Growth – “it could cost the Republicans the seat in the Senate for a generation,” according to the executive director of Republican Main Street Partnership, Sarah Chamberlain.

With $1.5 million on hand this year to support centrist Republican candidates – an estimated two-fold increase over the last cycle – the Main Street Partnership has always faced an uphill battle in terms of fund raising and the outward passion of its more activist opponents. But it has a winning record going head-to-head, such as in 2004, when it supported the incumbent Pennsylvania senator, Arlen Specter, for re-election. Although outspent more than 2-to-1 by the Club for Growth, which backed Rep. Pat Toomey – now its executive director – Mr. Specter won the primary and then the general election with 70%. He subsequently shepherded Mr. Bush’s Supreme Court nominees to confirmation.

These party divides are also at work in a lesser-noticed Michigan congressional primary in August, which pits an incumbent Republican congressman, Joe Schwarz, against Tim Walberg, who is being supported by the Club for Growth and Michigan Right to Life. Here too, the intra-party debate is viciously partisan, with attack Web sites dedicated to the claim that “Joe Schwarz is a liberal.” Columnist Hans Zeiger opines, “Joe Schwarz does not belong in the Republican Party. He is a Democrat at heart and a socialist at mind.”

Sometimes cooler heads prevail. In March, the fiscally conservative Hispanic Democratic congressman, Henry Cuellar, faced a primary challenge from a former congressman, Ciro Rodriguez, who accused the incumbent of being too close to the president. Without a Republican challenger, the primary was the election. This time, the Club for Growth took the unprecedented step of backing Mr. Cuellar, bolstering its credentials as a principled rather than simply partisan organization. Mr. Cuellar won.

The hunt for heretics does not help a party in a time that is ripe for realignment. And yet the temptation seems to be too much for true believers to resist. Their disproportionate influence in low-turnout primaries is yet another indication of the gap that exists between professional partisans and the general public.

If Mr. Lieberman were to fall victim to such liberal Democratic cannibalism, a successful independent candidacy could help bring some needed clarity. If centrist candidates like Mr. Lieberman are rejected by their party’s most activist elements and still run, the independent label may increasingly become a place of refuge for the sensible center, an opportunity to forge a new national consensus.