Out of one decision, we get two great elections.

Florida Gov. Charlie Crist is opting to run for the U.S. Senate and in just a matter of days his primary with Marco Rubio started turning into a battle for the soul of the Republican Party. On top of that, with no incumbent in the governor’s race, Crist’s decision sets up a splendid race for the job Crist will leave behind.

Besides being wide open, the 2010 Florida governor’s race now should draw disproportionate national interest. Why? For the same reason that any state election typically draws national interest: Redistricting.

In every populous state, Washington will closely watch the 2010 elections because of their significance for congressional redistricting. In only a few, however, will the attention be squarely on the race for governor. In Texas, for example, the governor’s race is important, but most of the attention will be on the state House of Representatives, where Republicans are clinging to a 76-74 advantage.

In contrast, in the Sunshine State, the GOP has lopsided edges in both houses of the legislature, making the governor’s race the only realistic chance that Democrats have to influence the redistricting process.

Here’s my list of the five governor’s races that will affect congressional redistricting the most, with Florida coming in at No. 2.

No. 1 — California

In 2001, Democrats in California controlled redistricting, but chose to draw a map that mostly protected incumbents. They doubted they could wring more seats out of the Golden State without spreading the party’s voters too thin. Coming off Barack Obama ’s 24-point romp in California, don’t expect a repeat of that.

Democrats see as many as eight California Republican House members as vulnerable. If the Democrats can’t win those seats in 2010, they’re likely to try to win some of them by drawing a more aggressive map. They may also want to protect the party’s most vulnerable incumbent in the state, Jerry McNerney .

The state legislature is out of reach for the GOP, but the good news for Republicans is that they have a real shot at preserving some influence over redistricting by winning the governor’s race. State Insurance Commissioner Steve Poizner and former eBay executive Meg Whitman both have tons of self-funding potential (former congressman Tom Campbell is also in the race). Democrats have three imperfect candidates in Attorney General (and former governor) Jerry Brown, San Francisco Mayor Gavin Newsom and Los Angeles Mayor Antonio Villaraigosa.

No. 2 — Florida

Control of redistricting in Florida is an even bigger prize than you might expect because of the large number of potentially vulnerable congressman in the state. Democrats have three junior incumbents in marginal districts in Suzanne Kosmas, Alan Grayson and Ron Klein . Republicans have to worry that the Democrats will make another run at their three Cuban-American congressmen in South Florida, as they did in 2008, or will target Vern Buchanan or Tom Rooney . With a retirement, either the seats of Republican Bill Young or Democrat Allen Boyd would be top targets for the opposing party.

The governor’s race looks like an absolute tossup, with Chief Financial Officer Alex Sink the top candidate for the Democrats and Attorney General Bill McCollum the early front-runner for Republicans.