U.S. Senator Tom Harkin, a veteran Iowa Democrat, said on Saturday he will not seek re-election in 2014, putting at risk what was considered a safe Democratic seat. (Jonathan Ernst/Reuters)

When Sen. Tom Harkin (D-Iowa) announced Saturday that he wouldn’t seek a sixth term, Republicans rejoiced at the possibility of picking up the seat in 2014.

But, that joy soon gave way to political reality — the likelihood of a primary between conservative, tea-party-aligned Rep. Steve King and a more establishment GOP figure such as Rep. Tom Latham or Lt. Gov. Kim Reynolds. And, just in case Republicans thought they might avoid that sort of primary fight, King released this statement Saturday night: “Iowans now have a real opportunity to elect a true Constitutional conservative.” (In case you were wondering, he is referring to himself.)

Iowa is one of a handful of states where Republicans have the opportunity to pick up Democratic Senate seats in 2014 — West Virginia, Minnesota and Alaska are the three other obvious examples. However, the party also seems likely to face a political dynamic that has plagued it for each of the last two elections: the most conservative candidate wins the primary, but then loses the general election.

The names are now famous — actually infamous — in Republican strategist circles: Sharron Angle (Nev.), Christine O’Donnell (Del.), Ken Buck (Colo.), Richard Mourdock (Ind.) and Todd Akin (Mo.). Over the past four years each of them took races that were somewhere between slam dunks and should-have-wons and managed to lose them. Take those five seats and put them into Republican hands and the Senate is a 50-50 partisan split.

In 2014, just like in 2010 and 2012, the Senate map favors Republicans. Twenty-one Democrats have to stand for reelection, compared with 14 Republicans. And, many of the Democrats seeking reelection will do so in places like South Dakota, Louisiana and Arkansas — not exactly friendly territory for the president’s party.

And yet, caution is the watchword among the Republican smart set; twice bitten, thrice shy — or something like that.

A look at some of the top Republican pickup opportunities suggests wariness is the best approach for party strategists. Aside from South Dakota, where popular former Republican governor Mike Roundslooks like he will have clear shot at Sen. Tim Johnson (D), potentially problematic primaries loom.

In West Virginia, polling suggests Rep. Shelley Moore Capito (R) would be a favorite to replace retiring Sen. Jay Rockefeller (D), but some conservatives believe she is insufficiently loyal to party principles to allow her to be their standard-bearer. Soon after she announced her candidacy in late 2012, Chris Chocola, president of the Club for Growth, lambasted Capito’s “long record of support of bailouts, pork and bigger government,” adding: “That’s not the formula for GOP success in U.S. Senate races.”

So far, no Republican opponent for Capito has emerged. Rep. David McKinley, who is being recruited by conservatives to run, seems disinclined to do so, but has yet to publicly endorse Capito. If McKinley says no, conservatives seem likely to look elsewhere.

In Alaska, where Sen. Mark Begich (D) is imperiled due to the strong conservative tilt of the state, Republican Joe Miller made the rounds on Capitol Hill last week, according to National Review’s Bob Costa. Miller, a tea party favorite who ousted Sen. Lisa Murkowski (R) in 2010 only to lose to her in the general election, is considering a second Senate run — a bid that would put him in a primary fight with Lt. Gov. Mead Treadwell (R), who has a far more moderate reputation in the Last Frontier.

Then there is Minnesota, where Sen. Al Franken (D) will stand for a second term in November. The Republican field is largely unformed at the moment with names like Reps. Erik Paulsen and John Kline being mentioned. But, a recent Public Policy Polling survey— an automated poll with a Democratic lean — showed that one candidate would crush all comers in a Republican primary: Rep. Michele Bachmann.

Bachmann, a 2012 presidential candidate and beloved figure within the tea party, would fare far less well in a general election matchup with Franken, however. According to the poll, Franken would take 54 percent to 40 percent for Bachmann.

And, it’s not just in Democrat-held seats where divisive primaries won by conservatives with little crossover appeal could be a problem. The retirement of Sen. Saxby Chambliss in Georgia, for example, will almost certainly set off a massive GOP primary scramble. If an ideological purist who can’t reach to the middle is nominated, it’s possible that Democrats with the right candidate — Rep. John Barrow, perhaps? — could have a real shot in what is a Republican-leaning state. (Did anyone think Democrats were going to win in Missouri and Indiana last fall?)

What happens in these Senate races will not only go a long way toward determining which party controls the chamber in 2015, but also will set the tone for the Republican presidential primary in 2016. What kind of the party do Republicans want for themselves: ideologically pure but without governing power or less rigid ideologically but with genuine control?