For months, Hillary Rodham Clinton's Democratic rivals watched in frustration as she smoothly pivoted away from her 2002 Senate vote authorizing military action in Iraq.

Now her vote on a measure designating Iran's Islamic Revolutionary Guard Corps as a terrorist organization has raised new concerns among anti-war activists. And it has given her opponents, especially Barack Obama, a fresh chance to question the former first lady's judgment on matters of war and peace.

While last month's nonbinding measure won strong bipartisan support in the Senate, some war foes have suggested such a designation would be tantamount to giving President Bush authority to invade Iran.

Clinton, a member of the Armed Services Committee, was the only Senate Democrat running for president to vote in favor of the measure. Joe Biden and Chris Dodd opposed it. Obama skipped the vote to campaign in New Hampshire but issued a statement that day saying he would have voted no.

In defending her vote, Clinton said Thursday that it was important to have leverage in dealing with Iran's government.

"There was nothing in that resolution that gave President Bush or anyone any authority to go to war," she said at an apple orchard in Canterbury, N.H. "We want leverage so if we can ever convince the Bush administration to actually negotiate, they've got some sticks to use at that table."

"That's what I would do," Clinton said. "Negotiate with them, no conditions, but with some sticks we could use to try to get leverage to move them in the direction we want."

Clinton has taken heat for her vote since last month's Democratic debate in New Hampshire. Long-shot rival Mike Gravel said he was ashamed of her, and John Edwards questioned why she hadn't learned from her 2002 Iraq vote, as he had from his.

No one has hit harder than Obama, whose long-standing opposition to the Iraq war has been a central theme of his candidacy. He broke from his practice of not criticizing the New York senator directly, granting interviews and writing a biting op-ed article in New Hampshire's largest newspaper Thursday condemning the vote as dangerous and reckless.

"Sen. Clinton says she was merely voting for more diplomacy, not war with Iran. If this has a familiar ring, it should. Five years after the original vote for war in Iraq, Sen. Clinton has argued that her vote was not for war — it was for diplomacy, or inspections," Obama wrote in the Union Leader newspaper. "America needs a leader who will make the right judgments about matters as grave as war and peace, and America needs a leader who will be straight with them," he said.

Obama's campaign also released a Web video Thursday marking the fifth anniversary of the Senate Iraq vote, chastising Democrats who gave a "blank check" to President Bush and suggesting a war with Iran might come next.

Clinton and her aides vigorously dispute the notion that her vote on the Iran measure signaled a willingness to engage militarily with Iran. Since her vote, Clinton has signed onto legislation by Virginia Democratic Sen. Jim Webb requiring Bush to seek explicit congressional authorization to invade Iran.

Clinton spokesman Howard Wolfson also noted that Sen. Dick Durbin, Obama's Illinois colleague and strong supporter, voted in favor of the Iran resolution.

"Senator Obama's campaign has stalled, and he is therefore abandoning the politics of hope and employing the same old attack politics instead," Wolfson said. "If Senator Obama felt so strongly about this, why didn't he speak out or vote against it at the time?"

That, in turn, prompted a retort from Obama spokesman Bill Burton, who noted that Clinton began her presidential quest promising a conversation with voters.

"Whatever happened to the politics of 'Let's chat' and 'Let's have a conversation?"' he said. "Obviously, they find it irritating to answer tough questions on important issues like Iran."

Clinton's vote has also begun to resonate on the campaign trail, where she's worked hard throughout the campaign to convince anti-war Democrats that she would move quickly to end the Iraq war as president.

In Iowa this week, she sparred with a voter who quizzed her about it, and she suggested the question had been planted by a rival campaign. She later apologized.

There could be other problems ahead. Eli Pariser, who heads the anti-war online group MoveOn.org, said the organization had problems with the Iran measure.

"Our members are concerned about any legislation that even appears to give President Bush authority to expand the mess in Iraq into Iran," Pariser said. "MoveOn members are looking for presidential candidates who won't make the tragic mistake we made in Iraq again."

Even so, Peverill Squire, a political scientist at the University of Iowa, said he didn't think Clinton's vote would hurt her chances in the long term.

"In terms of how people evaluate her, one of Clinton's strengths is that she's tough. This reinforces that impression of her," Squire said. "If there were any political calculations on her part, they were for the general election, rather than the primary season."