PITTSBURGH, Pennsylvania (CNN) -- After a weekend of playing defense about his "bitter" comments, Sen. Barack Obama went on offense Monday against his Democratic rival and the presumptive Republican nominee.

Sen. Barack Obama participates Sunday in the Compassion Forum at Pennsylvania's Messiah College.

Obama mocked Sen. Hillary Clinton for throwing back a shot of whiskey in front of TV crews over the weekend and said she must think she's "doing me a favor" by attacking him and toughening him up for a fall race against Republican Sen. John McCain.

"I'm sure that Sen. Clinton feels like she's doing me a favor because she's been deploying most of the arguments the Republican Party will be using against me in November and so it's toughening me up. I'm getting run through the paces here," Obama told The Associated Press' annual meeting.

Clinton has been hitting Obama hard after he referred to some small-town Pennsylvanians as "bitter" people who "cling to guns and religion" at a fund-raiser last week.

Obama later said the remarks were badly phrased but accurate. Watch Obama try to clarify his comments »

On Sunday, Clinton called the comments "elitist, out of touch and frankly, patronizing," and added, "You know, the Democratic Party, to be very blunt about it, has been viewed as a party that didn't understand and respect the values and the way of life of so many of our fellow Americans."

Obama opened his remarks to the AP making light of what is being called "bitter-gate."

"I know I kept a lot of you guys busy this weekend with the comments I made last week. Some of you might even be a little bitter about that," he said to soft laughter.

But his offensive began Sunday night when he mocked Clinton for acting like "Annie Oakley ... packin' a six-shooter" in her attempts to connect with gun owners.

He was referring to Clinton's efforts over the weekend to appeal to Second Amendment supporters by hinting that she has some experience of her own pulling triggers. "I disagree with Sen. Obama's assertion that people in our country cling to guns and have certain attitudes about trade and immigration simply out of frustration," she began.

"You know, my dad took me out behind the cottage that my grandfather built on a little lake called Lake Winola outside of Scranton and taught me how to shoot when I was a little girl," she said.

Asked Sunday when she last fired a gun or attended church services, Clinton said the query was "not a relevant question in this debate." Watch Clinton face questions on gun use »

Obama blasted Clinton Sunday shortly before the two appeared at Faith in Public Life's Compassion Forum at Messiah College in Grantham, Pennsylvania.

"Shame on her," he told a Steelton crowd. "I expected this out of John McCain," Obama said in a decibel higher than his everyday stump tone.

"But I've got to say, I'm a little disappointed when I start hearing the exact same talking points coming out of my Democratic colleague, Hillary Clinton. She knows better. Shame on her." Watch Obama target Clinton »

On Monday, he attacked her for what happened at an Indiana campaign stop over the weekend.

Saying too many candidates are giving voters only rhetoric, the senator from Illinois added, "They'll promise you anything. They'll even give you a long list of proposals. They'll even come around with TV crews in tow and throw back a shot and a beer."

With the national media present, Clinton drank a beer and chatted with voters. After ordering her beer, the bartender asked, "You want a shot with that Hillary?" After some deliberation, Clinton settled on a shot of Crown Royal, a Canadian whiskey.

Responding to Obama's remarks, Clinton spokesman Phil Singer said, "With all due respect, this is the same politician who spent six days posing for clichéd camera shots that included bowling gutterballs, walking around a sports bar, feeding a baby cow and buying a ham at the Philly market (albeit one that cost $99.99 a pound). Sen. Obama's speeches won't hide his condescending views of Americans living in small towns."

Clinton heard a few boos Monday as she continued to criticize Obama.

"I understand my opponent came this morning and spent a lot of his time attacking me," she said at the same forum where Obama launched his assault.

The crowd responded with audible grumbles, and a few in the hall shouted, "No!"

Clinton continued, "I know that many of you, like me, were disappointed by the recent remarks he made." Watch as CNN's Candy Crowley offers a recap of the 'bitter' dispute »

This time, a louder, sustained chorus of "No!" emanated from the audience.

"I am well aware that at a fundraiser in San Francisco he said some things that many people in Pennsylvania and beyond Pennsylvania have found offensive," she said.

Again, she was met with jeers, which the Clinton campaign said came from Obama supporters. E-mail to a friend

CNN's Peter Hamby and Chris Welch contributed to this report.

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