Congressional incumbents almost never face serious primary challenges. Those who do are often weakened by scandal. But there are occasional primary challenges that are battles for the soul of a political party.

We have four such primaries coming up in the next two months that expose the tensions in both parties.

In the House, for the past 20 years, fewer than two incumbents on average each election have lost in primaries (leaving out redistricting years, when incumbents are often pitted against each other in primaries). In the Senate, only three incumbents have lost in primaries since 1982.

Often, candidates do not face any primary opposition. Take for example, Michigan, where all 14 incumbents in 2004 were unopposed in the primary.

In 2006, however, Republicans Chris Cannon and Lincoln Chafee and Democrats Joe Lieberman and Daniel Akaka face serious challenges.

When this column appears, we will know the results of the Utah GOP primary between Rep. Cannon and John Jacob. Jacob’s main charge against Cannon is that he is too favorable toward immigration. A Jacob win will buck up the argument of House Republicans that illegal immigration is a motivating issue for voters, and more House campaigns will reflect that theme in the fall. Pro-immigration Republicans in the House and Senate might face primary challenges in 2008.

In Rhode Island, before Sen. Chafee can worry about a tough general election battle, he must beat Club for Growth candidate and Mayor Stephen Laffey in the GOP primary. Polls show the race is very close.

Chafee may get significant support from independents, who may vote in the primary, because there is no longer a race on the Democratic side, but Republican voters favor Laffey.

The Club for Growth has backed a number of free-market conservative challengers against moderate Republicans. It has had an effect, making it more difficult for moderates to oppose their party on certain issues. The Club almost beat Pennsylvania Sen. Arlen Specter with its candidate, then-Rep. Pat Toomey, in the 2004 GOP primary. A Laffey win would be the group’s biggest triumph yet.

On the Democratic side, Sen. Akaka faces an unexpected primary challenge from Rep. Ed Case. While this is not a battle for the soul of the national party, it highlights a changing Hawaii political landscape.

The Democratic establishment there has been a force since Hawaii was admitted as a state in the 1950s. Sen. Dan Inouye stands at the top of this powerful party. But Republican Linda Lingle was able to win the governor’s race running against Democratic excesses, and demographic changes on the island have made Native Hawaiians and the World War II generation less powerful. Case, nearly 30 years Akaka’s junior, represents this new generation.

Finally, Sen. Lieberman is in serious trouble over his support for the war in Iraq. The left of the Democratic Party is angry at those who originally voted for the war but apoplectic at Lieberman for his continued support of the war and President Bush. Challenger Ned Lamont has tapped into this sentiment, and the Connecticut polls have closed up considerably.

Liebermann has even flirted with the idea of running as an independent if he loses the primary, but that course would likely be disastrous. To win, Lieberman has to win the primary.

While our political system is heavily weighted against primary challenges, the strength of these four challenges may indicate a greater role for primaries to “purify” both parties. Even the increased threat of primary challenges is likely to move moderates away from the center and into the arms of their own party leadership.

Fortier is a research fellow at the American Enterprise Institute.