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., who has campaigned with Obama on his six-day bus tour of the Keystone State.
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."
Rendell appeared on the program with Casey, who said Obama is "certainly the underdog in our state."
Obama spoke at a town hall Monday morning and visited a gas station later in the day to highlight the rising cost of a gallon of gas. As part of his campaign in the state, Obama is airing an ad in which he says he's not beholden to the oil companies and offers his plan for energy independence.
"I don't want to make promises I can't keep. So I don't promise that the minute I'm elected suddenly the gas prices drop a buck," Obama told the town-hall audience. "That's not gonna happen but what I can promise is we can go after those windfalls."
Asked by a voter at a town hall event in Lancaster, Pa., about his vote for the Patriot Act reauthorization, Obama said some provisions were needed.
"I'll give you an example: prior to the Patriot Act you could not wiretap a phone that wasn't land-based. Now think about it.
Nobody uses a land-based phone anymore. Certainly (terror suspects) aren't going to be using an old dial-up phone, right? So it made sense then to change that law and there were some other changes that made sense."
He added: "I want everybody to understand I taught constitutional law for 10 years. I take the Constitution very seriously," he added.
Joshua Dratel, a criminal defense lawyer who has worked on terrorism cases before and after the Sept 11, 2001 attacks, said Obama's description "is not correct."
"I've been involved with wiretaps on people with cell phones, both terrorism, and non-terrorism cases, going back to the 1990's.
He's confusing two separate concepts," said Dratel.
The Patriot Act did, for the first time, allow a judicial authorization in terrorism cases for what is often referred to as "roving wiretaps" - listening in on calls from a particular person even if they switch cell phones and SIM cards to try to evade authorities.
Prior to the Patriot Act, federal agents could get the exact same roving wiretaps for criminal terrorism investigations that they could get for organized crime and anti-drug investigations and other serious federal crimes. The Patriot Act, and a subsequent expansion passed in 2002, allows the government to get "John Doe" roving wiretaps under the Foreign Intelligence Surveillance Act (FISA) which do not have the same protections for innocent non-targets as criminal surveillance orders.
"I think that's what he probably meant, but what he said is not accurate," said Dratel.
Lawyer Seth Taube, a former federal prosecutor who specializes in corporate finance cases, agreed with Dratel.
"The Patriot Act introduced roving surveillance wiretaps of cell phones, by a person rather than a number, but the ability to tap a cell phone existed beforehand," said Taube.
Asked later in the day what he meant by saying the federal government could not wiretap cell phones prior to the Patriot Act, Obama said: "The way that the laws were written at the time you had to have a fixed location."
