Obama said Sunday that Coakley's opponent, Republican state Sen. Scott Brown, would march in "lockstep" with Washington Republicans.
Election of Coakley, a Democrat, is seen as critical to the party, as a Brown victory would give the GOP the 41 Senate votes needed to block healthcare reform.
"Where we don't want to go now is backwards," Obama told supporters at Northeastern University in Boston. "We've got so much work left to do. ... I can't do it alone. I need leaders like Martha by my side so we can kick it into high gear, so we can finish what we've started."
Obama said Coakley would fight for working people while Brown wants to go back to what the president called the failed agendas of the Bush administration, The Boston Globe reported.
Organizers estimated about 1,500 people packed a gym with an additional 2,500 viewing the event in an adjacent room.
Before the president took the stage, Ted Kennedy's widow, Victoria Reggie Kennedy, spoke.
"As Teddy would say, Jan. 19 is the date, Massachusetts is the state and Martha Coakley is our candidate."
White House spokesman Robert Gibbs said Obama accomplished what he had hoped to during the trip.
Gibbs, speaking aboard Air Force One, said the White House is not focusing on how it might get healthcare reform passed if Coakley loses.
"We think Martha Coakley is going to win this race," he said.
A rally for Brown in Worcester drew about 2,200 people with about 1,000 in two more rooms.
"Friends and fellow citizens, I'm Scott Brown, I'm from Wrentham, I drive a truck and I'm asking for your vote," said Brown, who likes to point out he has a pickup truck with 200,000 miles on it.
His rally drew celebrities and sports figures, including former Red Sox pitcher Curt Schilling and Doug Flutie, the former Boston College and NFL quarterback.
One sign said, "The president may be in Boston, but the real people of Mass. are here with Scott Brown in Worcester."
The race has been surprisingly close in a state where Democrats outnumber Republicans 3-to-1.