Assemblywoman Cathleen Galgiani is about to find out whether the rainbow flag can stretch to cover the Central Valley.

And same-sex marriage supporters talking about taking the initiative road to overturn Prop. 8’s ban on gay unions will be watching closely to see what happens.

Galgiani, a Democrat who lives in Livingston in Merced County, came out as a lesbian this week in an interview with the Stockton Record.

The timing is interesting, since the 47-year-old waited until she was termed out of her 17th Assembly District seat before making the announcement. But Galgiani already has announced that she plans to move back to her old hometown of Stockton to run in the redrawn 5th State Senate District in 2012.

While Stockton’s a lot more urban than Livingston, which is known for chickens, cows and sweet potatoes, neither city is a hotbed of liberalism. And when Galgiani joins the Legislature’s LGBT caucus, she’ll be the only member from a county that doesn’t touch the Pacific Ocean.

(The current members are Assemblyman Tom Ammiano and state Sen. Mark Leno from San Francisco; Assemblyman Rich Gordon from San Mateo; Assembly Speaker John Pèrez and Assemblyman Ricardo Lara in Los Angeles; and state Sen. Christine Kehoe and Assemblywoman Toni Atkins in San Diego).

Despite the same-sex marriage zealots who argue that a vote for 2008’s Prop. 8 was a declaration of homophobia, there are folks opposed to gay marriage who would think nothing of voting for a gay or lesbian.

Still, though, a look at the Prop. 8 numbers across California show just how tough a road any gay politician faces not only in the Central Valley, but also anywhere in the rural reaches of the state.

A map of the vote shows that except for the Northern and Central California coastal counties and the tiny outposts of Mono and Alpine counties, the state was awash with Prop. 8 supporters. And the counties in the state’s agricultural midsection was where support for the gay marriage ban was strongest: San Joaquin, 66 percent; Stanislaus, 68 percent; Merced, 71 percent; Fresno, 69 percent; Tulare, 75 percent; Kern, 75 percent.

The new state Senate district, which includes Stockton, Lodi, Tracy and Manteca, is a lot less rural than Galgiani’s current Assembly seat, but it’s not going to be a breeze for any Democrat, gay or straight.

But while Galgiani said she decided to come out now to send a positive message to young people struggling with their sexual identity, there’s also a whiff of politics in the decision.

With a year to the 2012 general election, it’s a good time to get the issue, ah, out of the closet. Since Galgiani apparently wasn’t hiding her sexual orientation from her close friends and newspapers were beginning to ask questions, it was only a matter of time before it became public knowledge, possibly under less than optimal circumstances (Hello, Roy Ashburn).

(As an aside, there’s really only one answer any politician, gay or straight, should have to a question about sexual orientation: “That’s none of your damn business.” In the nicest possible way, of course).

Now Galgiani can control the message and turn it into a relative non-issue by the time serious campaigning begins in the state Senate race.

While plenty of people, many of them academics, will argue that California has moved beyond considering sexual orientation in elections, you might want to consider the number of out gays and lesbians holding statewide office or even winning primaries for those jobs.

While the state has mostly rejected the abrasive “anti-gay” style of campaigning, there still are plenty of ways for candidates to “remind” voters that they’re straight and their opponents aren’t. Campaign appearances and mailers featuring the wife and kids are a nice touch, as are reminders that the opponent represents (wink, wink) “San Francisco values.”

But gay rights advocates know just how important it is for Galgiani and other LGBT candidates to make a good showing in Central California. As they found out in the Prop. 8 campaign, having Gavin Newsom greet voters in the Castro District and sending same-sex marriage supporters to campaign at the downtown Berkeley BART station can’t make up for collecting only 25 percent of the vote in Tulare and Kern counties.

If a measure to overturn the gay marriage ban ever gets back on the state ballot, supporters are going to need more votes from Central California. And the way to get those votes is to show people there that gay and lesbian candidates are their neighbors. Like every politician, they’re anxious to support the issues important to the community, whether it’s schools, water and agriculture or, in Galgiani’s case, high-speed rail for the Central Valley.

Galgiani will be a pioneer in her upcoming state Senate race, which isn’t necessarily a good thing for any politician. But win or lose, the race will have implications that will resound across the state.