Click here to watch "Exactly."

Pennsylvania residents witnessed perhaps the most vitriolic days of the Democratic primary race over the weekend, with Barack Obama and Hillary Rodham Clinton slapping back and forth at one another in campaign stops across the state.

The Obama campaign has largely steered clear of negative advertising over the course of the primary contest. But, as CBS News reported, over the last several days "Obama has begun to fight fire with fire. In ads, campaign mailings and rhetoric, Obama is getting much tougher as this campaign (perhaps) nears some sort of finish line."

Four new TV ads released since late last week demonstrate the fierce tone that the Obama team has adopted in Pennsylvania, with the Illinois senator hitting Clinton on health care, ties to lobbyists and what he characterizes as her negative tactics.

"Dime" was crafted in response to a Clinton spot accusing Obama of being dishonest about his relationship with lobbyists and oil company executives. "Across Pennsylvania families are struggling. What's Hillary Clinton's answer? The same old politics -- misleading negative ads," an announcer charges. He goes on to defend Obama's record of fighting oil companies and repeat the claim that Obama is "the only candidate who doesn't take a dime from oil company PACs or lobbyists."

In "Afford," Obama seeks to shift attention back to one of the few policy disagreements between the two Democratic candidates: a mandate for universal health care coverage. "What's [Clinton] not telling you about her health care plan?" an announcer asks. "It forces everyone to buy insurance, even if you can't afford it."

"Reason" touts a litany of Pennsylvania newspaper endorsements for Obama but does not refrain from simultaneously digging at his rival. "The Post-Gazette calls Hillary Clinton's attacks the 'cynical responses of old politics,'" the ad says, quoting various editorial boards in order to deliver its attacks. Meanwhile, the spot hits on Obama's message of change with lines such as: "The Patriot News says Barack Obama 'offers real change in the White House.'"

Obama's newest ad, "Exactly," released on Sunday, represents perhaps his most striking repudiation of Clinton's candidacy. The announcer lambastes the New York senator for, "in the final hours..., launch[ing] the most misleading and negative ad of the campaign." Again maintaining that Obama doesn't take money from special interests, the spot claims that "Clinton has raised millions from PACs and lobbyists -- more than any candidate in either party," as names of special interest groups that have contributed to her campaign scroll across the screen. Hammering home its message, the spot concludes: "Eleventh-hour smears paid for by lobbyists' money; isn't that exactly what we need to change?"

In addition to what's on TV, voters are also seeing "dueling" mailers from the two campaigns on trade issues.