Rudolph Giuliani, the former New York City mayor confirmed he is running for the US presidency in 2008.
There had been doubts among Republicans about whether Giuliani was serious about a White House run but he said "Yes, I'm running," twice during an interview on the Larry King Live show on Wednesday.
He also said he is not confident a positive change of events can occur in Iraq but said that a withdrawal of US troops would only create future problems.
When asked what led him to pursue the presidency, he said: "I think I can make a difference. I believe that the country needs leadership."
On Iraq, Giuliani said: "I'm confident that we have to try to make a turnaround and we just can't walk out, and that it is critical to us that things get to the point in Iraq that we have some degree of stability and not the way they are now," he said.
National polls have shown Giuliani leads eight other Republicans, in part because of his steely and comforting leadership amid the chaos that followed the 2001 attacks that brought down New York's most visible landmark.
Giuliani became one of the Republican party's most popular figures after 9/11.
Earlier this month, Giuliani filed a statement of candidacy with the Federal Election Commission establishing a committee to explore a presidential bid, but he stopped short of making a formal announcement.
However, he faces an uphill battle winning over conservatives who wield considerable influence in party primaries because of his stance on some social issues, including his support for gay rights and abortion rights.