New district

The battle for this new district pits Democrat Kyrsten Sinema, a former state senator, against Republican Vernon Parker, the former mayor of Paradise Valley. Both parties have invested heavily in the race, though a Democratic super PAC has tipped the TV ad war balance of power slightly in the Democrat's favor.

California 30: Rep. Brad Sherman (D) vs. Rep. Howard Berman (D)

Merged district

One of the nastiest House races of 2012 is between two incumbent Democrats due to redistricting and the state's new primary system, in which the top two finishers advanced to the general election. Rep. Brad Sherman has at least one advantage over Rep. Howard Berman: He represents more of the redrawn Sherman Oaks-area district. Sherman won the primary with 42.4% of the vote compared with 32.4% for Berman. The remaining votes were split among five candidates. The race hit a nasty patch last month, when the two men shouted at each other at a public forum. Both have received heavyweight endorsements. Independents and Republicans could play the decisive role.

California 31: Rep. Gary Miller (R) vs. Bob Dutton (R)

Seven-term incumbent Rep. Gary Miller faces a tough challenge from a fellow Republican in a race also determined by California's new primary system. Miller's opponent is Bob Dutton, a state senator and businessman. Miller represents none of this redrawn district, while Dutton represents much of it in the state legislature. Money could play a decisive role: At the start of October, Dutton had only $58,000 in the bank; Miller's war chest held $816,000 on October 17. Miller edged Dutton in the June top-two primary, 26.7%-24.8%, with the remaining 49% divided among four Democrats. Those Democratic voters will likely decide whether Republican Miller returns to Washington for an eighth term.

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. His background could appeal to the district's growing Hispanic population. But Bono Mack has survived strong challenges before and has a 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 an advantage 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 e-mailed 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 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, lost by nine points. The key difference this time is that Rivera has been dogged by scandal and ethics issues. The district is still Republican-friendly and Garcia entered 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 had expected would be the case 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 in Barrow's new district is 44%, compared with 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 with Anderson, who emerged weakened from a competitive primary.

Illinois 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 August, Rep. Patrick Kennedy told CNN that his longtime friend and colleague was suffering from "serious depression -- deep, deep depression."

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.

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 when favorite son Barack Obama is heading the ticket for the other party. Walsh has made headlines with a number of controversial statements, including his assertion that medical science has advanced to the point where abortions are never necessary to save a woman's life. Democrat Tammy Duckworth, a decorated Iraq war veteran, led in fundraising and entered the final stretch with an advantage.

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