With soaring speeches and knee-capping commercials, the Democratic presidential candidates blanketed Pennsylvania yesterday in a final paroxysm of campaigning for the largest haul of votes left in their long nomination battle.

In the final hours of the longest single-state focus since the campaigns left Iowa, Sen. Hillary Rodham Clinton and Sen. Barack Obama each aired ads that tacitly targeted their opponent.

The exhausted survivors of the long Democratic race bounded around the state for a final crowded day of appearances climaxed by dueling rallies in basketball arenas at opposite corners of the state.

The stakes were high in the most expensive and most-watched Pennsylvania presidential primary in history. Trailing in the delegate count and most national polls, Mrs. Clinton badly needed a victory to sustain her cash-strapped campaign and convince Democratic Party leaders that she was the better candidate to take on Republican Sen. John. McCain.

The last-minute Clinton ad was a thematic variant of her "3 a.m. phone call'' message, which aired in the Texas primary. The commercial features cascading images of national emergencies, from Pearl Harbor to Hurricane Katrina to Osama bin Laden, with the text: "It's the toughest job in the world. You need to be ready for anything -- especially now, with two wars, oil prices skyrocketing and an economy in crisis. Harry Truman said it best -- if you can't stand the heat, get out of the kitchen. Who do you think has what it takes?''

The Obama retort -- without mentioning Mrs. Clinton's name -- brought another of his frequent reminders that, in 2002, she had voted to authorize the war in Iraq.

"Who made the right judgment about opposing the war and had the courage and character to speak honestly about it. And who in times of challenge will unite us -- not use fear and calculation to divide us," a narrator says.

The ads were the latest shots in the increasingly negative and expensive volleying that has dominated the state's airwaves. They were accompanied by mailers and automated phone calls in which two candidates with largely similar policy positions exchanged charges on health care, trade, gun rights and other issues.

"He has gone sharply negative, and he is doing this to knock us out of the race," Howard Wolfson. Mrs. Clinton's spokesman, claimed in a mid-morning conference call with reporters.

Mr. Obama has outspent the increasingly cash-strapped Clinton campaign by a margin estimated at between two-to-one and three-to-one over the final weeks of the Pennsylvania campaign.

The latest federal statistics showed that the Clinton primary campaign had fallen in the red as of March 31, with more debt than cash available for pre-convention spending. Mr. Obama, on the other hand, had $42 million available for the primary.

Mrs. Clinton entered the state campaign with a significant preference poll lead, which contrasted with her deficits in national polls, in the number of delegates won in previous primaries and caucuses and in the popular vote.

Her lead in Pennsylvania has narrowed considerably, but the consensus of public polls continued to show her with a comfortable but not unassailable lead in the mid-single digits.

The real scoring will come with today's balloting. A big margin for either side could produce a clear victor, but in a closer race, the real winner might not be immediately apparent.

Any victory would be enough for Mr. Obama. Given her underdog status nationally, however, Mrs. Clinton needs a big win. How big? That could be a function of the perception of the beholders.

Overall, according to a tally by the Associated Press, Mr. Obama has lined up a total of 1,646 pledge delegates and committed superdelegates to 1,508 for Mrs. Clinton, with 2,025 needed for the nomination.

In the state's Democratic primary, which, unlike some other states, is restricted to registered Democrats, voters will have the opportunity to vote for the candidates themselves, and will also be faced with a choice of delegates.

For almost everyone except the delegates themselves, the only vote that counts today is the top-of-the-ballot Clinton versus Obama choice. The winner of that contest, and the winner's margin, will determine how many delegates the congressional district will produce for each candidate.

The separate delegate preference vote simply determines the order in which the delegate candidates will be chosen. Voters are free to cast votes for one nomination candidate at the same time at they same as the choose delegates supporting his or her opponent.

If a district has four delegates, for example, and one presidential candidate wins, 55 percent to 45 percent, that district will split its delegates, two apiece. The same vote total in a five-candidate district would mean a 3-2 split. The delegate preference vote would decide which two or three of the aspiring Clinton or Obama delegates actually gets to make the trip to Denver. In addition, however, because of separate Democratic rules on gender and minority representation, the order of delegate finish may not be a straightforward guide to the winning delegates.

Different congressional districts award different numbers of delegates. Some have been awarded bonus delegates according to their levels of support for Democratic candidates in past elections. Some will produce as few as three delegates. On the other hand, Philadelphia's 2nd Congressional District has nine delegates, the most in the state.

While Mrs. Clinton is favored in the statewide popular vote, this pattern of bonus delegates could end up favoring Mr. Obama because the delegate-rich districts occur disproportionately in the eastern part of the state where he is expected to be more competitive than in the west or center.

Three of the four congressional districts that will elect seven delegates are in the Philadelphia region. The 14th District, which includes Pittsburgh, is the only seven-delegate district outside that area.

Rep. Chaka Fattah, an Obama supporter who represents the 2nd District, predicted in an interview this month that his candidate would win as much as 80 percent of the vote there, an outcome that would yield a 7-2 delegate split for his candidate.

Under that scenario, that one district, would make up for the one-delegate advantages Mrs. Clinton might reasonably expect in five five-delegate districts scattered through the parts of the state where she is seen as stronger.

Another factor that would counter Mrs. Clinton's strengths in the state is that most of the counties that have recorded the biggest registration gains in the state are in the southeastern region where Mr. Obama appears to be most competitive.

The weather is expected to be fair across the state today, setting the stage for the kind of big turnout that has occurred in state after state in the Democratic nomination battle.