Sen. Hillary Rodham Clinton confirmed Monday that as president she would be willing to use nuclear weapons against Iran if it were to launch a nuclear attack on Israel.

Clinton’s remarks, made in an interview on MSNBC’s “Countdown With Keith Olbermann,” clarified a statement she made last week in a Democratic presidential debate in Philadelphia. In that debate, Clinton, D-N.Y., said an Iranian attack on Israel would bring “massive retaliation,” without defining what the phrase meant.

In the interview Monday, Clinton affirmed that she would warn Iran’s leaders that “their use of nuclear weapons against Israel would provoke a nuclear response from the United States.”

She said U.S. allies in the Middle East were being “intimidated and bullied into submission by Iran,” raising the prospect of an “incredibly destabilizing” arms race in the region.

“I can imagine that they would be rushing to obtain nuclear weapons themselves” if Iran were to develop a nuclear arsenal, she said.

Clinton said it was vital that the United States create a new “security umbrella” to reassure Israel and its other allies in the region that they would not be threatened by Iran. She said she would tell them that “if you were the subject of an unprovoked nuclear attack by Iran, the United States, and hopefully our NATO allies, would respond to that.”

Clinton seeks tougher profile than Obama

Clinton’s hinting at a nuclear option last week set off a wave of commentary in political circles that she was seeking to position herself as a hawk as the primary campaign winds toward an end. Her opponent for the Democratic nomination, Sen. Barack Obama of Illinois, has said that he would not rule out any options if Iran were to become a nuclear power, but he has not explicitly said he would be willing use nuclear weapons.

Clinton’s remarks reflected the theme of her latest advertising in Pennsylvania, where Democratic voters go to the polls Tuesday with analysts in both camps saying she must win the state’s primary if she is to remain a credible candidate.

Obama leads Clinton by 1,655-1,513 among delegates won in primaries and caucuses, according to NBC News’ count, while she has the advantage, 262-238, among superdelegates, the party officials who attend the convention by virtue of their positions. But Clinton leads in Pennsylvania by 5 to 7 percentage points in most tracking polls released Sunday.

Clinton’s new ad says that only she has the experience and toughness to confront the threat of global terrorism, using a picture of al-Qaida leader Osama bin Laden in an echo of Republican attacks against Democratic Senate candidates in 2004.

Asked by Olbermann if the use of bin Laden in the ad was “waving the bloody shirt,” she said she was not indulging in Republican-style fear-mongering against Obama, who has been the subject of false rumors that he is Muslim.

“The fact is the next president will be sworn in at a time of very, very difficult world conditions,” Clinton said. “There’s nothing at all, in any way, inappropriate in saying look, presidents face the unexpected all the time. We don’t know what the next president will face.”

She said the Democratic nominee was sure to face such attacks in the fall campaign, and “we’re going to have to go toe to toe with John McCain on national security. We ought to get real about some of the big issues were going to face.”