White House Press Secretary Robert Gibbs on Wednesday acknowledged that President Barack Obama’s popularity has slipped since he took office a year ago but was optimistic that his supporters will rally behind him and the Democratic Party come November.

The lagging excitement among Obama’s core backers could have serious consequences for Democrats, who need a large turnout in the midterm elections to stem losses in the House and the Senate. Liberals have been concerned about the president’s decision to send 30,000 more troops to Afghanistan and to jettison a public insurance option form the health care reform bill, among other issues.

But Gibbs predicted that voters will come around. “There’s a lot at stake,” Gibbs said.

Gibbs said the president still has no plans to travel to Massachusetts to campaign for state Attorney General Martha Coakley (D), who is running in Tuesday’s special election to succeed the late Sen. Edward M. Kennedy (D-Mass.). Sen. Paul G. Kirk Jr. (D) is filling the seat temporarily.

National Democrats and their allies have been forced to pour millions into the special election, which Coakley had been expected to win easily but now has become increasingly tight.