Thomas Ferraro has a good report on the topic:
It is axiomatic that a strong presidential nominee can boost the chances for other party candidates, particularly those in close U.S. Senate races.
But the presidential candidate can also hurt those farther down the ticket.
While a single comment might not alter particular races, a lagging campaign could. Republican Representative Steven LaTourette of Ohio, who is retiring from Congress, defended Romney's remarks, but said they "don't help in swing districts like mine." "People were ready to throw Obama over, like dumping a boyfriend, and were ready to be courted by a new boyfriend," he said. "But now they're having second thoughts," LaTourette said.
Republican Representative Tom Cole of Oklahoma called Romney's remarks "an unfortunate choice of words," but predicted the comments would be "a one- or two-day story."
"The election is going to turn on the economy," Cole said. University of Virginia political scientist Larry Sabato, whose "Crystal Ball" blog closely tracks congressional races, said Romney's performance would be particularly influential in Senate races in Virginia, Connecticut, Montana, North Dakota and Florida.
"Scott Brown can't survive much more undertow in Massachusetts," he said. George Allen, the Republican Senate contender in Virginia, "depends on a Romney win," Sabato added.
"As I go through the states, I'd say Romney's performance will help to determine most of the close Senate contests," he said in an email interview.
"It's going to be very difficult for Republicans to take over the Senate if Romney doesn't capture the White House. That's a different evaluation than a year ago when the GOP looked to be a good bet to grab the Senate."