California 36: Rep. Mary Bono Mack (R) vs. Raul Ruiz (D)

Rep. Mary Bono Mack, who is seeking an eighth full term in the House, is a frequent target of Democrats hoping to pick off a Republican in a Democratic district. This year, she finds herself fending off a strong challenge from Democrat Raul Ruiz, a Mexican-American, Harvard-educated physician with a compelling personal story. His background could appeal to the district's growing Hispanic population. But Bono Mack has survived strong challenges before and has a relatively moderate voting record. She also prevailed over Ruiz in her first one-on-one matchup, the June "top-two" primary in which they appeared together on the same ballot.

California 44: Rep. Laura Richardson (D) vs. Rep. Janice Hahn (D)

Merged district

Redistricting has forced another pair of Democratic incumbents to face off this November. Rep. Janice Hahn won a July 2011 special election to replace retiring Democratic Rep. Jane Harman. Rep. Laura Richardson was elected in a 2007 special election to replace the late Juanita Millender-McDonald. Hahn appears to have a clear advantage in the race due in part to an ongoing ethics saga for Richardson that has resulted in a reprimand and $10,000 fine by the House Ethics Committee for campaign finance violations involving her congressional staff. Hahn also enjoyed a huge fundraising and cash-on-hand advantage at the start of October. Hahn trounced Richardson in the primary.

Florida 9: Former Rep. Alan Grayson (D) vs. Todd Long (R)

New district

In just one term in Congress, Democrat Alan Grayson mastered the art of making headlines with his blunt and abrasive rhetorical style. During the debate over health care, he said the Republican health care plan was, "Don't get sick, and if you do get sick, die quickly." He also said on CNN that Republicans were "foot-dragging, knuckle-dragging Neanderthals." In a 2010 TV ad, he called his Republican opponent Daniel Webster "Taliban Dan." Although he quickly became a hero among liberals, Grayson went on to lose his bid for a second term by a staggering 18 points.

Grayson is running in a new district in the Orlando suburbs and his Republican opponent is Todd Long, an attorney, small businessman and conservative radio show host.

Florida 18: Rep. Allen West (R) vs. Patrick Murphy (D)

Rep. Allen West is a top target for Democrats The freshman Republican's sharp rhetoric during his first term has not endeared him to colleagues across the aisle. For instance, West last summer emailed Democratic National Committee Chairman Rep. Debbie Wasserman Schultz that she was "the most vile, unprofessional and despicable member of the U.S. House of Representatives." West's Democratic opponent is Patrick Murphy, a businessman and executive with a construction firm. West has a huge fundraising advantage but Murphy has been running ads and the race is very competitive.

Florida 26: Rep. David Rivera (R) vs. Joe Garcia (D)

The race in Florida's southernmost congressional district is a rematch of 2010, but the dynamics could not be more different. Republican incumbent David Rivera was elected in the Republican wave two years ago. Democrat Joe Garcia, a former Miami-Dade County Democratic party chairman who lost by nine points. The key difference this time around is that Rivera has been dogged by scandal and ethics issues. The district is still Republican-friendly and Garcia enters the final stretch in better position than two years ago.

Georgia 12: Rep. John Barrow (D) vs. Lee Anderson (R)

Democratic incumbent John Barrow is fighting for his political life in this heavily Republican district, which is far better than Republicans expected at this point when they drew the district's boundaries. According to an analysis by the Cook Political Report, President Barack Obama's 2008 vote percentage in Barrow's new district is 44%, compared to 55% under the old lines. Despite the unfavorable political landscape, Barrow has made the race competitive. He has far outpaced his Republican opponent, state Rep. Lee Anderson, in fundraising. He also has been in general election mode from the start compared to Anderson, who emerged weakened from a competitive primary.

Ilinois 2: Rep. Jesse Jackson Jr. (D) vs. Brian Woodworth (R)

Rep. Jesse Jackson Jr.'s biggest obstacle to a ninth full term in Congress might be himself. Jackson has been on medical leave since June for an undisclosed ailment, which his office later described as a "mood disorder." In mid-October, CNN confirmed that Jackson was the subject of a federal investigation over possible financial improprieties. In late October, Jackson released a campaign robocall to constituents saying he was undergoing treatment for "several serious health issues" and asked for "your continued patience as I work to get my health back." Republican opponent Brian Woodworth, an attorney and university professor, has criticized him for leaving the district unrepresented. Despite his absence, Jackson is expected to win fairly easily.

Illinois 8: Rep. Joe Walsh (R) vs. Tammy Duckworth (D)

Freshman Republican Joe Walsh is high on the list of most endangered GOP incumbents. Walsh barely won his seat in 2010 and redistricting has made it more Democratic. He has the additional misfortune of running in a presidential election year with favorite son Barack Obama heading the ticket for the other party. Walsh also has made headlines with various controversial statements, including most recently one that medical science has advanced to the point where abortions are never necessary to save a woman's life. Democrat Tammy Duckworth, decorated Iraq war veteran, led in fundraising and enters the final stretch with an advantage.

Illinois 10: Rep. Robert Dold (R) vs. Brad Schneider (D)

Republican Robert Dold won this Democratic-friendly district in the Republican wave of 2010, replacing fellow Republican Mark Kirk, who ran for the U.S. Senate. The redrawn district is more Democratic-friendly, but Dold has kept the race competitive. His Democratic opponent is businessman Brad Schneider, who emerged battered from a competitive primary. Dold entered the final month of the campaign with a huge cash advantage but Schneider has benefited from sizable TV ad buys from the national Democratic party and from a pro-Democratic super PAC, but he still trails the overall TV ad dollars invested by Dold, the national Republican party and the U.S. Chamber of Commerce.

Iowa 3: Rep. Tom Latham (R) vs. Rep. Leonard Boswell (D)

Merged district

Iowa was one of 10 states to lose seats in Congress because of redistricting, setting up a member-on-member showdown between two veteran lawmakers in a merged Des Moines-area district. Democrat Leonard Boswell represented much of this new district in the late 1990s. Republican Tom Latham has a considerable fundraising advantage due in part to his close friendship with House Speaker John Boehner. The cash advantage is apparent on the airwaves, where Latham has outspent Boswell, even when counting the considerable assistance the Democrat has received from his national party. Boswell is no stranger to tight races. He survived the Republican wave of 2010. The presidential race will boost Democratic turnout, but his new district has many more "red" counties than the one he'd represented for the past 10 years. The race will be competitive.

Iowa 4: Rep. Steve King (R) vs. Christie Vilsack (D)