The impending retirement of Senator Tom Harkin of Iowa creates the opportunity for Republicans to pick up a seat that Democrats have held since 1985. But if they aren’t careful in selecting their candidate, it could represent another opportunity forsaken by the party.

The leading G.O.P. candidates are thought to be the two Republicans who currently serve in Iowa’s delegation to the House, Steve King and Tom Latham. Mr. Latham has repeatedly been elected from swing districts, and has a relatively moderate track record. According to the statistical system DW-Nominate, he was more moderate than about 80 percent of Republicans in the House based on his voting record through 2010.

Mr. King, in contrast, rated in the 90th percentile for conservatism among Republican representatives, according to the same system. He also has a history of drawing attention to himself with provocative statements on issues ranging from gay marriage to immigration.

Can we say how much of a difference this might make? The model that FiveThirtyEight uses to forecast Senate races relies in part on evaluating candidates’ ideology as determined by DW-Nominate and other systems.

DW-Nominate scores are on a scale that runs from negative 1 for an extremely liberal candidate to positive 1 for an extremely conservative one. A score of zero represents a candidate who is exactly in the middle of the ideological spectrum.

What matters for the FiveThirtyEight model is not what the candidate’s ideology is in an absolute sense, but how it compares to the voters in his state. In a red-leaning state like Indiana, for example, the ideal candidate is slightly right of center. But his advantage can be overcome if the Republican candidate is extremely conservative, while the Democrat is a moderate. Such an outcome transpired during the Senate race in Indiana in 2012, when a moderate Democrat, Joe Donnelly, beat a very conservative Republican, Richard Mourdock.

In Iowa, which sits right at the middle of the ideological spectrum, a candidate like Mr. King would have even more problems. Specifically, the model estimates that Mr. King would run a net of six percentage points weaker than Mr. Latham, controlling for other factors. For example, in a race in which Mr. Latham would be favored to beat the Democratic candidate by two percentage points, Mr. King would be expected to lose by four. Since the race in Iowa is otherwise likely to be close, this could easily cost Republicans a Senate seat.

Georgia

In Georgia, a Republican incumbent, Senator Saxby Chambliss, is retiring. For Democrats to win his seat, it would probably require a scenario more like the one that transpired in Indiana last year: Republicans nominate an extremely conservative candidate, while Democrats choose a competent and moderate one.

Essentially all Republican House members from Georgia, including Representatives Phil Gingrey, Tom Graves, Jack Kingston, Tom Price and Lynn Westmoreland, rate as being extremely conservative by DW-Nominate and other statistical systems (most have ratings similar to Mr. King’s). This is partly because Georgia’s Congressional districts are extraordinarily polarized, with a few overwhelmingly liberal districts and many very conservative ones. A safer bet for Republicans would be to nominate a candidate who had won office statewide before, such as their former secretary of state, Karen Handel.

Democrats could also face a difficult primary, however. Their electorate is likely to be about evenly divided between black and white voters. And although there was once a sizable contingent of conservative Democrats in Georgia, as in other states in the South, that has not been true recently; in 2008, just 11 percent of voters in the Democratic presidential primary identified themselves as conservative.

It could therefore be hard for voters in either party to reach a consensus on a candidate who had substantial appeal statewide. Because of Georgia’s partisan lean, especially in a midterm year, Republicans would be favored in such a lesser-of-evils situation, just as they would be if each party nominated strong candidates. But Democrats would have a chance if they emerged with the better candidate from the primary battles.

West Virginia

Republicans are also favored, in my view, to pick up a Senate seat in West Virginia, where John D. Rockefeller IV is retiring. Here, there are fewer potential problems with candidate quality. Instead, the Republicans already have a strong one in Representative Shelley Moore Capito, who has a reasonably popular and moderate track record.

Ms. Capito is likely to be challenged for her nomination by a candidate supported by the Club for Growth or other conservative groups. But is not clear that these challenges would have much of a chance at success. West Virginia is very conservative culturally, but it is also quite poor and moderate fiscally, not making it a good fit for libertarian-minded groups like Club for Growth.

Were such a challenge to succeed, however, it could be extremely risky for Republicans. West Virginia is among the only remaining states where there are still large numbers of conservative Democratic voters and conservative Democratic candidates. Democrats control the governorship, the Statehouse and the state Senate, and the former Democratic governor, Joe Manchin III, easily won his United States Senate races in 2010 and 2012.