* Republicans hold a 29-20 advantage
* Likeliest gain is in North Carolina
* Strong candidate in Washington state
By Colleen Jenkins
WINSTON-SALEM, N.C., Oct 11 (Reuters) - Democrats are on the defensive in this year's clutch of U.S. governors' races as four of the party's incumbent state chief executives step down compared with just one Republican.
The battle for the 11 governorships at play on Nov. 6 has received a fraction of the attention paid to the races for the White House and the U.S. Senate, where Democratic control is at risk.
Just half a dozen of the gubernatorial races are even competitive, and Republicans, who hold a 29-to-20 advantage over Democrats, appear to be in a strong position to increase their majority.
They are likely to pick up at least one post, according to recent polls and political analysts.
The Democratic and Republican governors associations are spending millions in the swing states of North Carolina, New Hampshire and Missouri, as well as in close contests in Montana, West Virginia and Washington state.
"Our goal is just to add to our majority of 29 governors, hopefully with a few seats," said Republican Governors Association spokesman Mike Schrimpf. "Any year that you're picking up states is a good year, especially in a presidential (election) year."
The most governorships the Republican Party has held is 34 in the early 1920s. Democrats have had as many as 39, according to National Governors Association data. One state, Rhode Island, currently has an independent governor.
The most likely gain for Republicans will come in North Carolina, where Democrats have held the state's highest office for the last two decades. Governor Bev Perdue decided not to seek re-election amid sinking popularity.
Republican Pat McCrory, a former Charlotte mayor who narrowly lost to Perdue in 2008, has consistently led this year over Democratic challenger Lieutenant Governor Walter Dalton.
Recent polls show McCrory up by double digits and getting strong support from Democratic and independent voters.
"I think what likely will be Dalton's undoing will be people voting for (Barack) Obama for president and McCrory for governor," said Steven Greene, an associate professor of political science at North Carolina State University.
McCrory has "been able to very effectively portray himself as a moderate conservative, sort of a pragmatic get-things-done candidate," Greene said. "That always sells well in North Carolina."
Montana, New Hampshire and Washington also have outgoing Democratic governors, and the outcomes there are far from certain.
Former Democratic U.S. Representative Jay Inslee has a slight edge in Washington over state Attorney General Rob McKenna, who is viewed as a strong candidate for the office that no Republican has won in 32 years.
Both candidates have raised more than $9 million in addition to the money outside partisan groups are pouring into the race.
It is unclear if ballot measures on legalizing same-sex marriage and the recreational use of marijuana will have any effect on the contest.
Both men are against the pot initiative but disagree on gay marriage, which Inslee supports and McKenna, a Catholic, opposes on religious grounds, his campaign said.
Controversial ballot measures don't drive additional turnout in presidential election years, but candidates' stands can make a difference at the polls, said Mark A. Smith, a political science professor at the University of Washington.
With Washington voters fairly split on gay marriage, "it's not really clear who gains and who loses from it," said Smith, adding that the governor's race is "still up for grabs."
Party officials said the races in Montana and New Hampshire are likely to be won by razor-thin margins, and voters there can expect a flurry of advertising in coming weeks as the cash-flush Republicans aim to pick up those seats.
Democrats are encouraged by the considerable lead Obama has gained in some recent polls in the battleground state of New Hampshire. Former state Senate majority leader Maggie Hassan is locked in a tight contest there with Republican Ovide Lamontagne, who lost prior bids for governor and the U.S. Senate.
Support for the president will help energize and solidify the Democratic base as Hassan's name recognition with voters continues to increase, said Colm O'Comartun, executive director of the Democratic Governors Association.
"That's a toss-up race," he said. "I think the president is starting to move substantially ahead in that state. And if that continues, that's going to be important."
Republicans look well-positioned to keep Indiana, where Governor Mitch Daniels is stepping down due to a term limit. In Missouri, Democratic Governor Jay Nixon has a sizeable lead even as Republican presidential candidate Mitt Romney draws higher polling numbers than Obama in that state.
The West Virginia race features a rematch between the candidates who squared off in a special election last year. Governor Earl Ray Tomblin, a Democrat, has raised nearly three times as much money as Republican opponent Bill Maloney, but voters in the state heavily favor Romney in the presidential contest.
Each party has two states in which their incumbent governors should prevail. Democrats are expected to hold on to Delaware and Vermont, and Republicans to North Dakota and Utah. (For a factbox on governors' races, click on (Editing by Xavier Briand)