Democrat Barack Obama has stepped up criticism of front-runner Hillary Rodham Clinton, sending Iowans a mailing noting that he opposed a measure that critics warn could lead to war with Iran and was supported by Clinton.

"Barack Obama opposed another Bush foreign policy fiasco," said the mailing, being sent to thousands of homes on Tuesday.

At issue is a measure in the Senate which declared Iran's Revolutionary Guard a terrorist organization, which Obama called "conventional" while campaigning in New Hampshire.

"What I've insisted is, we have to completely rethink foreign policy and diplomacy. ... We have to talk not just to our friends but also to our enemies. That includes Iran. That includes Syria," Obama said at a house party in Merrimack, N.H.

Clinton voted in favor of the Senate measure. She defended her vote over the weekend, saying she did so only after language which could be interpreted as an authorization of the use of force was removed.

Clinton sent a mailing to Iowa activists explaining her vote. It included comments from retired Gen. Wesley Clark and Illinois Sen. Richard Durbin siding with Clinton's vote.

Obama countered with his own mailing on Tuesday.

"Why is this amendment so dangerous?" Obama's mailing said. "Because George Bush and Dick Cheney could use this language to justify keeping our troops in Iraq as long as they can point to a threat from Iran, and because they could use this language to justify an attack on Iran as part of the ongoing war in Iraq."

The dispute goes to a fundamental disagreement between Obama and Clinton.

Obama routinely criticizes Clinton for voting in favor of authorizing the use of force in Iraq, a move he opposed even though he had yet to be elected to the Senate. He says that shows he has "judgment we can trust," while Clinton's judgment is flawed despite her experience in Washington.

Clinton has said only she would not have voted to authorize the use of force if she knew at the time what she now knows. She blames flawed intelligence.

In New Hampshire, Obama said the debate is representative of the campaign's larger differences.

"We have very good candidates on the Democratic side," he said. "The question, though, is whether you are going to nominate a candidate who is not just tinkering around the edges or can bring about real change."