WEST CHESTER, Pa. (AP) _ Barack Obama cast his Democratic presidential rival Saturday as a game-player who uses "slash and burn" tactics and will say whatever people want to hear, a sharp jab at her character in the final chapter of the pivotal Pennsylvania primary campaign.

Hillary Rodham Clinton implored voters to look beyond "whoop dee do" speechmaking and take a hard look at who's got the know-how to deal with the nation's burdens.

"I want everyone thinking," she declared, as if to suggest those backing Obama are not. Her implication was clear: She's substance, he's flash.

Altogether, the campaign for Tuesday's contest was dissolving into the sort of acrimony that makes party leaders long for the finish line, before the nominee is damaged in the fall. Obama's criticisms were direct, while Clinton's were oblique but unmistakable. At various times in the protracted contest, it's been the other way around.

He pressed the case against her at stop after stop, blunt words set against the bucolic backdrop of his train ride through the Pennsylvania countryside. For her part, Clinton struck back at a new Obama ad that criticizes her health care plan, telling a rally in York: "Instead of attacking the problem, he chooses to attack my solution."

Otherwise she stuck to her stump speech, something of a role reversal in a contest that has seen the New York senator going after her opponent while he has stayed measured.

The primary Tuesday follows a monthlong hiatus in voting.

Party officials known as superdelegates continued drifting toward Obama in that interim, increasing his edge in the race despite his series of gaffes, and that trend is bound to accelerate if he performs strongly Tuesday. Clinton is hoping a decisive win will put a stop to that. Polls have suggested she has a consistent if shrinking lead.

The New York senator spoke under a baking sun outside West Chester's 175-year-old fire house, striking a somber note about problems at home and abroad as she described the stakes for voters Tuesday. She asked them to think about the looming challenge of China, the restive Middle East, the trade imbalance and the debt burden.

"I don't want to just show up and give one of those whoop-dee-do speeches and get everybody whipped up," she said. "I want everyone thinking."

As she looked to exploit questions about his gravitas, Obama played on poll findings indicating unease with her veracity, and did so head on, with words that could easily slip into a Republican campaign ad should Clinton become the Democratic contender against GOP candidate John McCain.

In Wynnewood, several thousand supporters lined the tracks for the first stop on his daylong whistle-stop tour aboard a royal blue train car that pulled out of Philadelphia in late morning.

"I may not be perfect but I will always tell you what I think, and I will always tell you where I stand," he told the crowd. Then he spoke of his rival.

"She's taken different positions at different times on issues as fundamental as trade, or even the war, to suit the politics of the moment. And when she gets caught at it, the notion is, well, you know what, that's just politics. That's how it works in Washington. You can say one thing here and say another thing there."

He amplified the point at a later stop, in Paoli.

"Senator Clinton's essential argument in this campaign is you can't change how the game is played in Washington. Her basic argument is that the slash-and-burn, say-anything, do-anything special interest-driven politics is how it works.... Senator Clinton has internalized a lot of the strategies, the tactics, that have made Washington such a miserable place."

And again, farther down the tracks, in Downington: "She's got the kitchen sink flying and the china flying, the buffet is coming at me ... constant distractions, these petty, trivial, slash and burn, back and forth, tit for tat, politics."

Obama's comment sharply diverged from his recent above-the-fray attitude. Not since Clinton was the front-runner last year has her character been so sharply criticized. Then-candidate John Edwards portrayed her as a defender of a corrupt Washington system.

Casual without a tie or jacket, his shirt sleeves rolled up, the Illinois senator shook hands with conductors and rail-workers on the platform in Philadelphia and set off, tooting the train whistle. Flags and bunting draped the back. The train had four stops en route to his evening rally in Harrisburg.

Clinton organized five events across the state. Nick Clemons, her Pennsylvania director, said the campaign would deploy 5,000 volunteers to place phone calls and knock on doors. "This is not going to be a blowout race," he said. "We're looking for a win, and we think it's going to be a close race."

Obama's new TV ad on health care was to be shown statewide until the primary.

"What's she not telling you about her health care plan?" the ad asks. "It forces everyone to buy insurance even if you can't afford it, and you pay a penalty if you don't."

In accusing Clinton of wanting to penalize people who don't buy insurance, the ad does not explain how Obama would enforce his proposed mandate that children be covered, without penalties of his own.

Clinton and Obama have tangled over whether to require everyone to purchase insurance _ a rare bit of daylight between them on the issues. Clinton's health care plan includes a mandate to buy insurance and offers tax subsidies for people to be able to purchase it. Obama's plan does not include a mandate except for children, but provides assistance that he says would make coverage affordable.

Obama leads Clinton in overall delegates, 1,645-1,507, with 2,025 needed to win the nomination. Obama also has a thin lead in the popular vote that Clinton hopes to overcome before the final ballots are cast in June.

Associated Press writer Liz Sidoti reported from Paoli, Pa.