WASHINGTON (Reuters) - The huge U.S. budget deficit poses a national security threat and projects a “message of weakness” internationally, Secretary of State Hillary Clinton said on Wednesday as she urged Democrats and Republicans to tackle the problem.

Clinton, in a speech heralding a new “American moment” in U.S. foreign policy, said the Obama administration’s policy of greater engagement with the world has brought dividends such as a united front against Iran’s nuclear ambitions.

But she again stressed the corrosive effect of the mounting U.S. debt, which she said threatened the United States’ ability to chart its own course in the world and sends “a message of weakness internationally.”

“It poses a national security threat in two ways: it undermines our capacity to act in our own interest, and it does constrain us where constraint may be undesirable,” Clinton said in response to a question after her address to the Council on Foreign Relations think tank.

The U.S. budget deficit is projected to hit $1.5 trillion this year and has become a campaign issue ahead of November’s congressional elections in which President Barack Obama’s Democrats are trying to prevent big losses to Republicans.

Clinton in the past has said it was “personally painful” to see the yawning U.S. spending gap after her husband, former President Bill Clinton, ended his second term in 2001 with budget surpluses.

She said on Wednesday she had no wish to “relitigate” the reasons for the current budget crunch, but nevertheless pointed to policies enacted by Obama’s Republican predecessor George W. Bush.

“It is fair to say that we fought two wars (Iraq and Afghanistan) without paying for them and we had tax cuts that were not paid for either, and that has been a very deadly combination to fiscal sanity and responsibility,” Clinton said.

“So the challenge is how we get out of (it) by making the right decisions, not the wrong decisions,” she said.

‘NO FREE LUNCH’

Clinton, beaten by Obama for the 2008 Democratic presidential nomination, noted that in her current job she is “by law out of politics” and said the challenges ahead would need a bipartisan “detente” on key issues including a new nuclear arms treaty with Russia.

“Whether one is a Republican or a Democrat, a conservative, a progressive -- whatever you call yourself -- there is no free lunch and we cannot pretend that there is without doing grave harm to our country and to our future generations,” she said.

She again urged bipartisan support for the START treaty with Russia, which has not yet garnered the Republican backing it needs to secure Senate ratification.

The treaty, signed by Obama and Russian President Dmitry Medvedev in April, would cut the number of deployed warheads allowed to each of the former Cold War foes by about 30 percent.

U.S. officials have warned that any stalling on the treaty would set back the U.S. ability to monitor Russia’s nuclear weapons. Some Republicans have expressed concern that the treaty would limit U.S. missile defense options while others say Obama must also commit to modernizing the U.S. nuclear weapons that remain.

“It’s a political issue. I wish it weren’t,” Clinton said. “But I hope that at the end of the day the Senate will say something should just be beyond any kind of election or partisan calculation, and that everybody will pull together and we’ll get that START treaty done,” she said.