NEW HAVEN — Representative Christopher S. Murphy has been at once the other guy — the largely unknown subject of attack ads by his Republican rival, Linda E. McMahon — and the Democratic nominee who was presumed to be the heavy favorite in the Senate race to replace Joseph I. Lieberman.

But a funny thing has happened in Connecticut as the political world has focused on Missouri, Massachusetts, Virginia and the few other states considered likely to determine which party controls the Senate. Battered by Ms. McMahon’s advertisements before he even became the nominee, Mr. Murphy has found himself on the defensive in a race that could add an unexpected wild card into the battle for the Senate.

Mr. Murphy, who defeated a 24-year incumbent, Representative Nancy Johnson, in 2006 to represent Connecticut’s Fifth Congressional District, says he is not surprised that polls now show a tight race in a state where Republicans have not elected a senator since Lowell P. Weicker Jr. in 1982. Several months ago, polls showed Mr. Murphy well ahead of Ms. McMahon in a hypothetical matchup.

“We’ve always known this race was going to be close,” Mr. Murphy said before a luncheon with voters here, adding that Ms. McMahon has spent approximately five times as much on television ads as he has. “And Connecticut has a history of pretty competitive statewide races, so I’m confident I’m going to win, but I don’t think it’s going to be a blowout.”