Washington.– 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.