While conservatives quietly mobilized behind a state senator, Scott Brown, to fill the seat occupied by Mr. Kennedy for nearly 47 years, Democrats paid but slight attention to a contest that by every indication and by history should have been nothing to worry about.
Martha Coakley, the attorney general and Democratic Party candidate, barely campaigned in the weeks after winning her primary on Dec. 8.
The vastly different responses of the two parties contributed to a confluence of events that fundamentally altered the course of what should have been a routine special election.
By the time Ms. Coakley’s campaign and Democratic officials noticed that things were not right in Massachusetts — after reading an outside group’s poll on Jan. 9 that showed Mr. Brown holding a 1 percentage point lead — the fire, as one White House official put it, was out of control. The Tea Party reinforcements had arrived, and a conservative group from Iowa started running commercials here portraying Ms. Coakley as a big spender who would raise taxes, a powerful issue with independents.
“It was a classic case of everybody getting caught napping,” David Axelrod, a senior adviser to the president, said in an interview. “This guy knew exactly what he was doing. He’s an appealing candidate. Pleasant guy. He’s smart. He tapped into an antipolitician sentiment.”