My priorities have always been my faith, my family and my country, and at this stage in my life, and after nearly 12 years of public service in Florida and Washington, it's time I return to Florida and my family.Some reports are saying that Martinez has issues at home that he needs to attend to. Others think Martinez made the move to help Crist, though it's not entirely clear as to how it will benefit him. With Martinez gone, Crist can appoint someone who definitely won't run in the next election, and will help Crist with his campaign efforts.
But Martinez had already endorsed Crist for his Senate seat.
Since the governor is in charge of naming an appointment, Gov. Charlie Crist could easily put himself into the seat by ensuring his replacement as Governor, Lieutenant Governor Jeff Kottkamp, would give the Senate position to Crist before Crist gave Kottkamp the governorship. Then, Crist could run as an incumbent in the next election against foe Marco Rubio.
But in the past, that hasn't been the wisest political move. In 1977, Minnesota Gov. Wendell Anderson of Minnesota appointed himself to a Senate seat by giving his governorship away to lieutenat governor Rudy Perpich, who dutifully stuck Wendell in the seat. Wendell ruined his political career, and it's not likely that Crist would do the same.
This isn't the first time Martinez has left his job early. He resigned as chairman of the Republican National Committee after 10 months in 2007.
UPDATE: Politico says that Martinez was rumored to be interested in the open job as president of Florida State University, his alma mater. Martinez denied the rumors.
NPR has a good round-up of what's happened to appointed Senators (who were formerly governors and otherwise) later in their career.