Barbed exchanges about Iran between the two leading Democratic presidential candidates are matched by their mutually antagonistic legislative approaches to the subject on Capitol Hill.

Sens. Hillary Rodham Clinton (D-N.Y.) and Barack Obama (D-Ill.) have intensified their legislative action since a Sept. 26 vote Clinton cast in favor of a resolution labeling the Iranian Islamic Revolutionary Guards Corps a terrorist organization. Both candidates are trying to persuade Democratic primary voters that they will stand firm against any Bush administration planning for pre-emptive war against Iran.

For Clinton, the issue is particularly sensitive. She has come under withering attack by her opponents for voting for the Iran resolution offered by two strong allies of President Bush’s war policy, Sens. Jon Kyl (R-Ariz.) and Joe Lieberman (I-Conn). Her opponents say the vote will help the administration make its case for war with Iran, an argument she strongly rejects.

Clinton has counterattacked aggressively on the issue. A week after the vote, she signed a Webb bill stating that funds would not be available for war with Iran unless the White House gets explicit authorization from Congress. On Oct. 29, Clinton co-sponsored a resolution by Senate Majority Whip Dick Durbin (D-Ill.) stating that any “offensive” military action against Iran needs congressional approval.

On Nov. 1, Clinton signed a letter to Bush with 28 other Democratic senators and one Independent expressing “serious concerns” about “provocative” statements from the White House about military intervention in Iran.

“We wish to emphasize that no congressional authority exists for unilateral military action against Iran,” the letter said.

Obama did not sign the letter and he has not co-sponsored the Webb or Durbin measures. Instead, the day the Democratic letter was sent to the White House, he floated his own resolution stating that the use of force against Iran is not covered by the 2002 measure Congress approved authorizing the Iraq war.

Obama is implicitly highlighting his long-standing concerns with the Iraq authorization, which has become a central point of his criticism of Clinton, who voted for the 2002 resolution.

Obama’s office said the senator has long supported Webb’s original proposal, but in the wake of the “potentially slippery slope” caused by Kyl-Lieberman, the candidate thought it was “important to have some statutory language that addressed his concerns.” An aide in Obama’s Senate office said the senator wanted language that went further than the letter.

Webb said he has “no doubt” that Obama supports his approach.

Clinton’s presidential rivals have used her Kyl-Lieberman vote to portray her as incapable of learning from what they say was her mistaken vote on Iraq. The Iran vote took center stage in a Democratic debate last month in Philadelphia, with all Clinton’s rivals criticizing her and accusing her of acquiescing to Bush’s wishes on Iran.

Former Sen. John Edwards (N.C.) went as far as to say “she voted to give George Bush the first step in moving militarily on Iran — and he’s taken it.”

Obama called the vote “a continuation of the kind of foreign policy that rejects diplomacy and sees military action as the only tool available to us to influence the region.”

Clinton argued that her rivals were “missing the point” and noted that Durbin also voted for it. She would go further than both the resolution and the letter, she added, saying, “I am prepared to pass legislation with my colleagues who are here in the Congress to try to get some Republicans to join us, to make it abundantly clear that sanctions and diplomacy are the way to go. We reject and do not believe George Bush has any authority to do anything else.”

The Senate is unlikely to act this year on either the Webb bill or the Obama resolution, and neither appears to have sufficient support to attract the 60 votes that would probably be needed to clear the upper chamber.

Sen. Ben Nelson, the conservative Democrat from Nebraska who sits on the Armed Services Committee, said it is “generally understood” that the White House would need congressional authorization if it wanted to launch an attack on Iran.

“I don’t think there’s any reason to get ahead of the process,” Nelson said when asked whether Congress needs to reaffirm the process through legislation.

The prospects for action could change next year if either Clinton or Obama wins the nomination and Democrats see political advantage in scheduling a vote on the issue.

Intensifying the Iran debate next year, however, could allow Republicans to paint Democrats as soft on a regime that the White House says is determined to acquire a nuclear weapon.

“That’s just sheer politics that is sending a wrong signal to an out-of-control regime,” Sen. Lindsey Graham (R-S.C.) said of the Democrats’ legislative approaches.

Philippe Reines, a spokesman for Clinton, dismissed suggestions that Clinton’s recent legislative actions on Iran were motivated by concern over her vote for the Kyl-Lieberman resolution.

“She signed onto Sen. Webb’s bill because of concerning rhetoric emanating from the White House,” said Reines. He also pointed to Clinton’s April co-sponsorship of a bill to tighten restrictions on American-controlled companies from doing business with Iran.