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Mr. Brown ran about even with his Democratic opponent in Mr. Frank’s seat in the 4th District. In both that district and the 5th District, Cook analysts downgraded the races from “solid Democratic” to “likely Democratic,” though Mr. Wood still rates both Mr. Frank and incumbent Rep. Niki Tsongas as safe.

“We are running candidates that are a serious and viable alternative to the cast of characters that we have in Congress,” said Jennifer Nassour, executive director of the Massachusetts GOP, citing Mr. Frank’s upstart Republican opponent, 35-year-old Marine reservist Maj. Sean Bielat. “We’re going to see these races tighten up even more.”

Ms. Nassour argued that people in the Bay State are “sick and tired of one-party rule,” and said more than 50 percent of voters are unaffiliated and up for grabs.

Mr. Bielat’s surprise challenge has led Mr. Frank, who as head of the House financial services panel was a chief author of Democrats’ Wall Street overhaul bill, to cut back on his campaign assistance to other Democrats. This year, the 15-term lawmaker has given just $35,000 to a dozen candidates compared, with $240,000 to 86 candidates in 2008, according to a Wall Street Journal analysis of campaign-finance statistics compiled by the Center for Responsive Politics.

A Republican source with knowledge of the campaign described that reality as a victory in and of itself, along with the fact that the party was able to recruit challengers in nine of the 10 Massachusetts House races this year after fielding only four candidates in 2008.

The Massachusetts Democratic Party did not return messages seeking comment.

Stephen Dinan contributed to this report.