WASHINGTON — The close race for majority control of the Senate comes down to whether Republican candidates in Massachusetts and Connecticut can win over President Barack Obama’s voters and Democrats from Indiana to Arizona can impress Mitt Romney’s GOP backers.

Ticket-splitting is vital to the prospects of Senate candidates in a half-dozen races in states that Obama and Romney are expected to win handily.

Democrats hold a 53-47 advantage in the Senate, counting the two independents, and must defend 23 seats to the GOP’s 10. The Republicans need a net of four seats to grab the majority if Obama wins and a net of three if Romney captures the White House and Paul Ryan as vice president breaks a Senate tie.

Connecticut
Former professional wrestling executive Linda McMahon, in her second Senate bid, is running even with three-term Democrat Rep. Chris Murphy in the Democratic-leaning state.

The wealthy McMahon is financing her ads, forcing the DSCC to spend $2 million and counting in a state that’s solidly in the Obama column. This past week, the Democratic committee bought an additional $650,000 in ads while the Democratic group Majority PAC invested more than $500,000 to air spots to help Murphy.

Indiana
Although Obama won Indiana in 2008, it’s unlikely this year as Romney seems a probable winner along with Republican Rep. Mike Pence in his gubernatorial bid.

Yet Democratic Rep. Joe Donnelly is in a close race with Republican Richard Mourdock, a tea party favorite who unseated six-term Sen. Richard Lugar in the May primary. Donnelly has played up his moderate voting record in the House as a contrast to Mourdock. The Republican famously said after beating Lugar that bipartisanship meant Democrats siding with Republicans and that winning meant he would “inflict my opinion on someone else.”

Indiana is a “conservative state but a state that looks for results, not strident partisanship, in the tradition of Richard Lugar and Evan Bayh,” Donnelly said in an interview.

Massachusetts
In Massachusetts, Republican Sen. Scott Brown also is talking bipartisanship in his race against Democrat Elizabeth Warren. Brown won a special election in January 2010 to fill the seat of the late Democratic Sen. Edward Kennedy, but this election he’ll likely face 700,000 to 800,000 more voters, many Democrats or independents who favor Democrats.

Polls in the state show Obama with a hefty double-digit lead over Romney, a former Massachusetts governor. The same survey shows Brown and Warren in a tight race.

Not surprisingly, Brown tells viewers in a recent commercial, “To me, creating jobs is more important than what party you belong to. That’s why one of the first votes I took as a senator was for a Democratic jobs bill.”

Montana; N Dakota
Montana and North Dakota are expected to go for Romney, but split-ticket voting could lift first-term Democratic Sen. Jon Tester and Heidi Heitkamp, respectively. Republicans and Democrats say both have run near flawless campaigns to make their races highly competitive against a strong GOP political headwind.

Arizona
The Democratic counterpoint to Connecticut is Arizona, where Democrat Richard Carmona, a former U.S. surgeon general, has surprised the GOP, riding a compelling up-from-the-bootstraps biography to a close race against Rep. Jeff Flake.

The National Republican Senatorial Committee recently purchased more than $130,000 of ad time, an investment that would have seemed unlikely two months ago.

Maine
In Maine, Republicans and GOP-leaning outside groups are running ads against independent Angus King, the former governor who is widely expected to caucus with the Democrats. Democrats are spending heavily on ads against Republican Charlie Summers. The Democratic candidate, Cynthia Dill, has the backing of state Democrats but has gotten little attention from national Democrats.

Missouri
Missouri remains a true wild card. Democratic Sen. Claire McCaskill, once considered the most vulnerable incumbent, got a fresh shot at re-election when Republican Rep. Todd Akin said women couldn’t get pregnant in the case of “legitimate rape.” Republicans, including Romney, called on him to quit the race, but he stayed in.