Hillary Rodham Clinton invoked images of Pearl Harbour and Osama bin Laden in a television advertisement that questioned Barack Obama’s ability to lead in a crisis, ending the six-week Pennsylvania primary to a contentious finish on Monday.

In her commercial, bin Laden, the mastermind of the Sept 11, 2001, attacks, is featured along with grainy images of the stock market crash of 1929, the attack on Pearl Harbor, the fall of the Berlin Wall and Hurricane Katrina. Clinton warned voters not to “take a leap of faith or have any guesswork” when they cast ballots on Tuesday. The Obama campaign accused her of employing “the politics of fear.”

With 158 pledged delegates at stake in Pennsylvania, the candidates raced from Scranton to Pittsburgh to Philadelphia to rally their supporters and win over a dwindling lot of undecided voters.

While Obama spent nearly twice as much on TV ads in the state, Clinton broadcast a new commercial that used historic images from critical moments in America’s past to ask voters who they could trust in the White House. It did not mention Obama, but closed with “Who do you think has what it takes?”

Obama, after three days of criticising Clinton with some of his sharpest language of the campaign, did not directly engage her. But his campaign quickly responded with an advertisement of its own that asked voters: “Who in times of challenge will unite us — not use fear and calculation to divide us?”

Pennsylvania has become a battleground in the fight for the Democratic presidential nomination, with the future of Clinton’s campaign most likely resting on the outcome. Even a wide victory by her would not overcome her deficit in pledged delegates or in the popular vote of states that have held nominating contests.

Obama’s supporters worked to drum up support, saying they were still hearing rumours that Obama was a Muslim. He said: “If they don’t vote for me, it should be because they think Clinton or McCain have better ideas. It shouldn’t be because they think that I am less patriotic or because they question what my religious faith is.”