Despite public criticism of Congress as a whole, few incumbents are defeated in general elections, and even fewer lose primary races against members of their own party.

But primary contests can be the only meaningful outlet for voter discontent in congressional districts that are gerrymandered, as so many are these days, to ensure that one party or the other will always win the general election.

Such lopsided districts dominate the list of 16 across the nation identified by CQPolitics.com as featuring contests for 2008 in which U.S. House incumbents face unusually vigorous primary challenges.

There are five Democratic incumbents facing serious primary challenges.

Rep. Daniel Lipinski, D-Ill., has not one but three primary challengers in the 3rd District, which includes chunks of Chicago and its Cook County suburbs to the west: Palos Hills Mayor Jerry Bennett, Cook County prosecutor Mark Pera and local attorney and Army reservist Jim Capparelli.

Rep. Albert R. Wynn, D-Md., is in a 4th District rematch against lawyer and political activist Donna Edwards, who took 46 percent of the vote against him in 2006 in a black-majority Democratic stronghold just outside Washington, D.C. There is a third candidate in the race as well: real estate broker George E. Mitchell. Last year, a different third candidate took just under 4 percent of the vote.

Rep. Edolphus Towns, D-N.Y., who represents the Brooklyn-based 10th District, could face a challenge from Kevin Powell, who filed a statement of organization with the Federal Election Commission (FEC) in early August but has not officially announced as a candidate. Powell identifies himself as “a political activist, poet, journalist, essayist, hip-hop historian, public speaker, and entrepreneur,” and was a cast member on the first season of the MTV reality-television show Real World.

Rep. Dennis J. Kucinich, D-Ohio, who is making his second longshot presidential bid, faces a contest closer to home in the 10th District, which encompasses west side Cleveland and its suburbs. Challenger Rosemary Palmer says Kucinich has missed too many important votes in his quixotic bid for the White House effort.

Rep. Steve Cohen, D-Tenn., is in a faceoff with airline executive Nikki Tinker, who came in second in the crowded, 15-person 2006 primary contest in the Memphis-based 9th District. Cohen is white, while the district is majority black. Although Cohen has liberal views and a history of receiving support from African-American voters dating to his long state Senate tenure, some black activists argue that black representation should be restored to the 9th District, which Harold E. Ford Jr. held before his unsuccessful bid for the Senate last year.