With organization strength of just 51 of the 100 members, party strategists are well aware that their majority can slip away with the loss of as little as one seat (if the 2008 Republican presidential ticket wins and puts a GOP vice president in the tie-breaking role as president of the Senate) or two, if the Democrats win the White House.

But Democrats have another part of the numbers game working strongly their way: In this election cycle, they are defending only 12 of their own seats to 21 defending by the Republicans.

Moreover, while the situation can change greatly in the long period before Election Day 2008, only one of the Democratic seats up next year seems certain to host a competitive race. That is in South Dakota, a usually Republican-leaning state, where the seat held by Democratic Sen. Tim Johnson appeared up for grabs even before he was struck in December by a serious illness.

The following is a roundup of the 12 contests for Democratic-held Senate seats in 2008, in state-alphabetical order. A roundup of contests for Republican-held seats was published earlier.

Arkansas

Incumbent: Mark Pryor (first elected in 2002)

CQPolitics rating: Democrat Favored

When Pryor scored his solid 54 percent to 46 percent victory in 2002 to unseat Republican Sen. Tim Hutchinson, he benefited from his own credentials as state Attorney General and a political legacy as the son of popular former Democratic Sen. David Pryor.

As he crafts an image as a Democratic centrist similar to that conveyed by his father, Pryor appears a solid favorite headed into his 2008 bid for a second term.

Arkansas, like other Southern states, has conservative overtones that caution against rating Pryor as a shoo-in at this early point. Though the home state of former President Bill Clinton, who carried it for the Democrats in both 1992 and 1996, Arkansas went for Republican George W. Bush in 2000 and 2004.

Yet Arkansas has swung less sharply in the Republicans’ direction than most Southern states, as underscored by Democrat Mike Beebe’s comfortable victory over Republican former Rep. Asa Hutchinson last year in the open-seat race for governor.

The state’s most popular Republican, and the one with the best potential to give Pryor a serious run for the seat, is Mike Huckabee, who became governor in 1996 and was forced to retire by the state’s term-limit law. But Huckabee, whose genial manner has enabled him to broaden his support beyond his base of socially conservative voters, appears determined to launch a bid for the 2008 Republican presidential nomination. — Rachel Kapochunas