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Did Mel Martinez Just Do the GOP a Favor?

Florida Senator Mel Martinez, a top potential target for Democrats in 2010, announced yesterday that he will not seek a second term. Florida thus joins Delaware and Kansas — where Sam Brownback is also expected to retire, possibly in preparation for a gubernatorial run — as probable open-seat races in the 2010 cycle.

Wise observers like Tom Jensen of Public Policy Polling are already concluding that this is in fact good news for the GOP. Martinez’ approval ratings are marginal; a Quinnipiac poll last month pegged his numbers at 42% approve and 33% disapprove, and a Strategic Vision poll was broadly similar — 47% approve, 41% disapprove. Public Policy Polling, whose approval ratings can be idiosyncratic, had more pessimistic numbers: 23% approve, 37% disapprove. On average, that works out to 37% approve, 37% disapprove, or almost exactly breakeven.

There are no hard and fast rules about this, because approval ratings depend heavily on question wording and are often not directly comparable to one another, but from having studied these numbers in the 2006 and 2008 cycles, the following general rules of thumb apply:

— If the average of candidate’s net approval ratings (his approval rating less his disapproval rating) is +20 or better, he is usually on track to win re-election in the absence of significant game-altering events. Caveat: significant game-altering events occur more often that you might think for US Senators, especially two years out from an election. Probability of retaining seat: 90-100%.

— If the average of a candidate’s net approval ratings are +10 to +20, it may be possible to defeat him with a superior campaign (Kay Hagan, 2008) and/or in a wave election (Democrats, 1994) without specific, game-altering events, although the odds are usually against it. Probability of retaining seat: 70-95%.

— If the average of a candidate’s net approval ratings are in the single digits — +1 to +9 — he is significantly vulnerable, and may be anywhere from a modest favorite to a slight underdog depending on the strength of the opposition. Probability of retaining seat: 45-75%.

— If the average of a candidate’s approval ratings are even or negative, he is usually no better than a toss-up against well-organized opposition, and often somewhat worse. Probability of retaining seat: 25-50%.

Martinez’ numbers had placed him right on the brink of the third and the fourth categories, implying that he was about even-money to retain his seat. Can his potential Republican replacements do better than that?

It depends, of course, on just who those replacements are. Generally, one of the big advantages that incumbents have — even relatively unpopular ones — is that they have an easy time raising funds; pretty much every incumbent senator running for re-election in 2008 had at least $5 million in his pocket, with the exception of a couple in non-competitive races in small states. New candidates rarely have access to that kind of capital.

…unless, of course, they are brand names like Jeb Bush, who is reportedly contemplating a run for Martinez’ seat. Bush left office in 2006 with approval ratings in the +20 range; they may have diminished slightly since then as a result of Bush Fatigue, but Floridians have little problem distinguishing Jeb from George W., even if that’s less true of the rest of the country. Bush, should he choose to run, will have most of the advantages that an incumbent usually has: capital, name recognition, organization, enough stature to deter primary challengers.

That is not to suggest that Bush would have a cakewalk into the Senate. He has his own baggage, and would be a fundraising magnet for Democrats. The most expensive senate race in 2006 was Hillary Clinton’s in New York, which brought in a collective total of about $40 million, and the most expensive in 2008 was Norm Coleman’s in Minnesota, which brought in a collective total of $35 million. Bush vs X. might be somewhere in that territory or even higher — perhaps as high as $50 million — and would almost certainly set the record for an open seat race.

Nevertheless, I think Martinez probably did do the Republicans a favor if their candidate winds up being Bush — or Charlie Crist, who like Bush could run with most of the advantages of an incumbent. If it is anyone other than those two specifically, on the other hand, the fundraising and organizational strength is a lot to give up. Moreover, the presence of an open seat may be more attractive to prospective Democratic challengers. Florida CFO Alex Sink, who was reportedly about to pull out of a prospective senate run in 2010, is now reconsidering, according to Marc Ambinder. It is hard to imagine a Robert Wexler not thinking long and hard about wanting to stick it to Bush.

The most cautious way to put it is this: Florida was one of the top two senate races in the country before Martinez’ announcement, and although the parameters now look a little different, it remains one of the top two now.

Nate Silver is the founder and editor in chief of FiveThirtyEight.