LANCASTER, Pa. – Democratic Sen. Barack Obama secured the backing of a second female senator on Monday as a top supporter of Hillary Rodham Clinton raised expectations for her rival in the upcoming Pennsylvania primary.

Freshman Sen. Amy Klobuchar of Minnesota, who recently voiced concerns about the rancorous tone of the Democratic primary, said she was forced to choose because she found remaining uncommitted difficult.

“Between Barack and a hard place, I chose Barack,” she told reporters in a conference call. “He’s able to dissolve the hard, cynical edge that has dominated our politics under the Bush administration.”

Klobuchar joins Missouri Sen. Claire McCaskill in backing Obama, who hasn’t fared as well with white female voters as Clinton has. Clinton leads Obama among white women 59 percent to 36 percent, according to exit polls of 27 competitive Democratic primaries held so far. Obama has won among white females in only Vermont, New Mexico and his home state of Illinois, while tying Clinton in Utah.

The two presidential candidates each have the backing of 13 senators, who are superdelegates to the convention.

Klobuchar’s decision follows last Friday’s endorsement from Pennsylvania Sen. Bob Casey Jr..

Clinton holds a double-digit lead in state polls, and Obama is trying to erase or at least cut into her advantage in the April 22 primary. Gov. Ed Rendell, who backs Clinton, said Monday that Obama was probably making up ground.

“We have a very strong lead, but I think that lead is going to shrink,” Rendell said on ABC’s “Good Morning America.”