If Barack Obama played it safe, he never would have challenged Hillary Rodham Clinton for the Democratic presidential nomination two years after leaving the Illinois legislature.

Once he won the nomination, he never would have planned a splashy tour of foreign capitals that might have typecast him as inexperienced and pretentious. He never would have tried to fill a football stadium and become the first candidate since John F. Kennedy to accept a nomination outdoors. In the general election, he would not have bet on states like Indiana, North Carolina and Virginia that had not voted blue for decades.

And in his first 100 days as president, he would not have sought $1 trillion from affluent Americans and a similar sum from businesses to finance health care, education and energy initiatives. All that while simultaneously trying to save the auto industry, revive financial markets, end the Iraq war and redouble efforts to battle Islamic extremists in Afghanistan.

Mr. Obama’s closest aides have long embraced that pattern, even if it still startles official Washington. As his campaign manager, David Plouffe, put it last fall, “We are always better off on the high wire.”