MARTINEZ PULLS A PALIN…. Sen. Mel Martinez (R) of Florida still has a year and a half remaining in his first and only term in office. When he announced his retirement in December, Martinez vowed to stay in office until his term ends.

Apparently, he’s changed his mind, and will check out of the Senate earlier than planned.

Florida Sen. Mel Martinez (R), who had previously announced he would not seek re-election in 2010, has now decided to resign his seat — a move that will allow Gov. Charlie Crist (R) to appoint a replacement to fill the remaining months of Martinez’s term, according to a source familiar with the decision.

The Martinez news, which was first reported by CNN, throws a fascinating wrinkle into the open seat race to replace Martinez.

The key angle here is that Crist will fill Martinez’s vacancy while seeking the same seat for himself. Crist could appoint himself to serve the remainder of Martinez’s term, but the governor has already said he wouldn’t. Since Crist is already the frontrunner to win next year’s open-seat contest, he’d be taking an unnecessary risk with such a move anyway.

As for why Martinez will only serve four-and-a-half years of his six-year term, the departing senator didn’t really say. He issued a statement this morning announcing the move, but in terms of an explanation, Martinez would only say that “it’s time” for him to “return to Florida and my family.”

It’s a rather cryptic explanation for going back on his vow to serve until the end of his term.

Nevertheless, Martinez’s move will not have any effect on the makeup of the Senate — he’ll serve until Crist names a temporary successor. There are five other Republican senators who will retire from the chamber at the end of next year, but for now, all of them will complete their terms in office.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.