Pittsburgh ? Hillary Clinton, facing another must-win vote, this one in Pennsylvania, used images of Osama bin Laden and a nation at war on Monday to present a last-minute display of toughness and sharpen a contrast with Barack Obama, who spent the day telling voters he is better positioned to bring about change.

Today’s Democratic presidential primary will bring to an end a bruising six-week period that has seen both candidates take blows over misstatements and other problems, heightening the anxiety of Democratic Party leaders that the ongoing primary struggle will hurt their eventual nominee, even while presumptive Republican nominee John McCain consolidates his gains, unifies his party and prepares for the fall.

With most analysts expecting a Clinton win today, both campaigns engaged in furious spin to manage the expectations of the public and party leaders. Obama conceded to a Pennsylvania radio station that he will probably lose but noted that he has significantly closed a 20 percentage point deficit with Clinton in the state. Clinton, for her part, said she does not expect to win by much and questioned Obama’s viability in the fall if he cannot win large swing states like Pennsylvania.

A frenetic day of cross-state appearances by Obama and Clinton marked the last full day of campaigning before an anticipated record number of Pennsylvania voters go to the polls for a primary, their ranks swelled by an increase in voter registration generated by the prolonged campaign. The race featured more than $16 million in spending primarily on TV ads, with Obama holding a 3-1 advantage over Clinton.

Both quickly snapped up opportunities for free TV on primary eve, no matter the sophistication level. Obama and Clinton taped an appearance on Comedy Central’s “The Daily Show” and Clinton was interviewed for “Countdown with Keith Olbermann” and “Larry King Live.” Both Democrats, along with McCain, also taped an introduction to World Wrestling Entertainment’s “Raw.”

Attempting to display toughness, Clinton’s campaign launched a final ad that displayed images of bin Laden, Pearl Harbor and the stock market crash while a narrator asks, “Who do you think has what it takes?”

Obama campaign officials labeled the ad “fear-mongering,” as they did in Ohio when Clinton unleashed a controversial TV ad that asked voters who was better prepared to handle a 3 a.m. emergency crisis call at the White House. And the Obama camp followed up with an ad posted on its Web site in which an announcer asks, “Who in times of challenge will unite us – not use fear and calculation to divide us?”

Even with a Pennsylvania win, Clinton still will trail in delegates and the nationwide popular vote as the Democratic presidential campaign enters the homestretch, a six-week rush that will give voters their last say in who wins the nomination.

By the time voting ends on June 3, more than 25 million Democrats will have voted in most of the country, likely giving a preference but not the nomination to Obama.