WASHINGTON (October 13, 2012)—Control of the U.S. Senate may turn on 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 Mr. Obama and Romney are expected to win handily.

These candidates are significantly outdistancing their parties' presidential nominees in polls, turning what should be an election-year rout into too-close-to-call contests.

With about three weeks to Election Day, Democrats hold a slight edge in keeping their majority in the Senate, but Republicans have a real shot if they can put together a string of wins in Democratic seats.