4. COLORADO 06: Rep. Mike Coffman (R) vs. Andrew Romanoff (D)
For a case study in how a politician can reinvent himself, look no further than Coffman, who was elected in a deeply conservative seat but was redistricted into a swing, diverse Denver-area district. He faces one of the most high-profile Democratic challengers of the cycle, former state House Speaker Andrew Romanoff, who has raised more than $2.5 million for the race. This is one of the few GOP-held seats where immigration is poised to become a major issue in the campaign, because the district is one-fifth Hispanic.
Coffman's district is the type of affluent suburban seat that a future Republican presidential candidate needs to win. Obama won 52 percent of the vote here in 2012, and if Coffman loses in a midterm year, it wouldn't bode well for the GOP's long-term prospects.
5. GEORGIA 12: Rep. John Barrow (D) vs. Rick Allen (R)
Can red-state Democratic senators like Mark Pryor and Mary Landrieu hang on to their seats in a tough environment? Ask Barrow, who's the last white House Democrat left in the Deep South. Barrow has managed to fend off numerous GOP opponents, despite representing a district where Obama only won 43 percent of the vote. He's won over enough Republican voters by breaking with the president on numerous issues, most recently emerging as the first Democrat to call for embattled Veterans Affairs Secretary Eric Shinseki's resignation. In the past, Republicans have struggled to recruit strong candidates in this conservative district, but businessman Rick Allen won an impressive 54 percent of the primary vote to avoid a runoff. If Barrow loses in November, it's a sign of a GOP wave, at least in the South — one large enough to sweep Republican Senate candidates to victory.
6. IOWA 03: Staci Appel (D) vs. GOP challenger
The seat is one of the true House bellwethers of the cycle — it's one of only 18 districts Obama carried in 2012 that's held by a House Republican. With popular GOP Rep. Tom Latham retiring, Democrats are excited about their prospects with former state Sen. Staci Appel, who's vying to become the first female member of Congress from Iowa.
This is one of the strongest Democratic pickup opportunities: Republicans face a crowded June 3 primary while Appel faces no Democratic opposition. Appel's a stronger candidate than her prospective GOP rivals, but she lost reelection to the state Senate against a tea- party-aligned candidate in 2010. The environment could be similar in 2014, with Republicans getting a boost by a strong showing from Gov. Terry Branstad at the top of the ticket.
7. MASSACHUSETTS 06: Rep. John Tierney (D) vs. Seth Moulton (D) and Richard Tisei (R)
In the last 22 years, no member of Congress from Massachusetts has lost reelection, but Tierney is one of the most threatened incumbents in all of Congress this year — thanks to his family's involvement in a gambling scandal. Despite representing a Democratic-friendly seat, he barely defeated Republican Richard Tisei, a former state Senate minority leader. Tisei is mounting a comeback this year and running in a more favorable political environment than in 2012. But first Tierney needs to get through the primary, where he's facing strong opposition from Harvard-educated Iraq War veteran Seth Moulton, who has outraised the congressman in three straight quarters. If Moulton wins the nomination, Democrats' chances of holding the seat increase markedly.