Open seats -- those being abandoned by a lawmaker either for retirement or a run for higher office -- are perennially the most likely to switch parties, as they represent the closest thing to a fair fight that you get in Congress. With the powers of incumbency a non-factor, open seats are more swayed by national environmental factors and, if the playing field is tilted to one party, have a tendency to all fall in the same direction. In 1994, for example, Democrats lost 22 of their 29 open seats in the Republican wave; in 2006, Republicans lost eight of their 21 open seats and did not flip a single one of Democrats' 12 vacancies. Given those numbers, it's no surprise that the recent retirements of Reps. John Tanner, D-Tenn., and Dennis Moore, D-Kan., have stoked some nervousness within Democratic Party ranks. Moore and Tanner are the eighth and ninth Democrats to say they will not run in 2010. That's not exactly a flood, especially considering that 12 Republicans have already opted out of reelection runs. (By this time in the 2008 cycle, 16 Republicans and five Democrats had announced plans to leave the House.) Are Moore and Tanner isolated cases or the start of an epidemic? There are 49 Democrats who sit in districts carried by GOP presidential candidate John McCain in 2008 and, among those, roughly 18 have held their seats for four or more terms. It's those folks -- Reps. John Spratt, D-S.C., Ike Skelton, D-Mo., and Marion Berry, D-Ark., to name three -- who will be critical to watch in the coming weeks. If they retire, an epidemic may well be declared. But you can bet that Rep. Chris Van Hollen, D-Md., chairman of the Democratic Congressional Campaign Committee, is doing everything he can to keep them in their seats. Below, you'll find our rankings of the five seats most likely to switch parties in 2010. 5. Virginia's 5th District (Democratic-controlled): Freshman Rep. Tom Perriello, D-Va., has got guts. After being pilloried by national Republicans for voting for President Obama's cap-and-trade proposal, Perriello voted last month for the president's health-care bill. Those votes almost certainly doom his election chances in this Southside seat; Republicans have found a top recruit in state Sen. Robert Hurt. 4. Illinois' 10th District (Republican-controlled): The decision of Rep. Mark Kirk, R-Ill., to run for the Senate puts this affluent North Shore district up for grabs. Both parties have competitive primaries, but whoever emerges on the Democratic side -- state Rep. Julie Hamos and 2006/2008 nominee Dan Seals are the leading contenders -- will have an edge in the general election, given the strong Democratic lean of the district. 3. Louisiana's 3rd District (Democratic-controlled): Races in Louisiana are notoriously slow to develop, but the underlying demographics of this seat -- 37 percent voted for Obama in 2008 -- do not bode well for a Democratic hold. Recruiting for this race is complicated by the fact that Louisiana is widely expected to lose a seat in the decennial redistricting, and this district appears most likely to go. 2. Louisiana's 2nd District (Republican-controlled): Rep. Joseph Cao, R-La., won the admiration of many Democrats nationally when he cast the lone GOP vote in favor of Obama's health-care plan. That's a step in the right direction for Cao, but it's a mistake to assume he will win reelection because of that vote. Always remember this when talking about Cao: McCain took 23, yes 23, percent of the vote in this district in 2008. 1. Delaware's at-large district (Republican-controlled): Republicans acknowledge privately that Rep. Mike Castle, R-Del., who is running for the Senate in 2010, is likely the only GOPer who could hold this seat. Delaware's strong Democratic lean and the candidacy of former lieutenant governor John Carney, a Democrat, make this seat ripe for pickup. A new independent poll conducted in Nevada shows that Senate Majority Leader Harry Reid, D-Nev., is totally known and not particularly well liked by the state's electorate, a dangerous combination for the four-term incumbent. Republican strategists quickly jumped on other numbers in the Mason-Dixon Polling and Research survey -- Reid's favorable number (38 percent) is outstripped by his unfavorable (49 percent), and he trails two Republicans, former state party chairman Sue Lowden and businessman Danny Tarkanian, in head-to-head matchups. But the most damning data point in the survey is that no voters said they were not familiar with Reid. What it means is that voters think they know Reid and don't like him. It's why his early advertising (more than $550,000) to date has focused on what he has done and what he can do for the state. Reid's best, and perhaps only, path to re-election is to turn the contest into a transactional affair. Reid does have a recent blueprint for that sort of campaign in Senate Minority Leader Mitch McConnell, R-Ky., who focused on what he had delivered for the state in his 2008 reelection race. McConnell, of course, had advantages Reid doesn't enjoy: He is from a state inclined to support his party, and his numbers, while not great, were not nearly as miserable as Reid's. A corollary to Reid's situation may be this year's New Jersey governor's race, in which the unpopular Jon Corzine, a Democrat, was not reelected despite a flawed challenger who ran a less-than-stellar campaign. 