The Senate amendment authorizing the president to label the Iranian Revolutionary Guard Corps a terrorist organization has become the central front in the battle for the Democratic presidential nomination.

Illinois Sen. Barack Obama's campaign has made New York Sen. Hillary Clinton’s vote for the resolution the focal point of its attacks — and so far, it’s working. Clinton has been forced to defend that vote in Iowa, where some voters believe she’s helped moved the U.S. closer to a war in the Middle East.

The amendment sponsored by Sens. Jon Kyl, R-Ariz., and Joe Lieberman, D-Conn., cited evidence that Iran is engaged in terrorist activities inside Iraq and elsewhere. The amendment passed 76-22, but only after Democratic senators removed language stating that U.S. policy should be to "combat, contain and roll back the violent activities" of Iran, and "to support the prudent and calibrated use of all instruments of United States national power in Iraq, including diplomatic, economic, intelligence and military instruments" to stop Iran.

Supporters said the amendment as passed does not justify military action, and even Obama-backer Dick Durbin said, "If I thought there was any way it could be used as a pretense to launch an invasion of Iran I would have voted no." But Obama and other critics claim the amendment still contains language that the Bush administration could use as justification for war.

In particular, Obama complains that the amendment makes the case for structuring the military in Iraq to counter Iranian influence, and states that it is "a critical national interest of the United States" to prevent Iran from exerting its influence inside Iraq.

In a memo released to the press, Obama foreign policy adviser Greg Craig writes that such language "sets forth an entirely new rationale for keeping U.S. troops in Iraq and, if need be, for attacking Iranian forces." Craig says supporting the amendment without President Bush’s assurance that he will not use it as the basis for attacking Iranian interests is "naïve" — a word Clinton once ascribed to Obama's view of diplomacy.

Essentially, Obama is equating the amendment with Clinton’s 2002 vote authorizing the use of force in Iraq. Clinton has repeatedly said that she believed she was voting merely for additional time for U.N. inspectors in Iraq, not a U.S. invasion.

The Iran amendment has no specific call for military action. But Obama, who rests much of his candidacy on his opposition of the Iraq resolution, asks how Clinton could support giving this president any sort of excuse after allegedly being burned in 2002 in the run-up to the Iraq war. The implication: Clinton is either too naïve or too hawkish to be the Democratic nominee.

The argument has gained credence in the blogosphere and Iowa voters, who are historically dovish. Earlier this month in New Hampton, one voter asked why he should elect Clinton when it seemed clear that she hadn’t learned her lesson from the Iraq vote.

Clinton’s testy exchange with the man made headlines, but she also sketched out her defense on the issue.

First, she pointed to a February speech in which she’d spoken out against giving the president any power to act against Iran without Senate approval, and noted that she’d signed on to legislation that would enforce that position. Second, she claimed that she voted for the Kyl-Lieberman amendment only after the more bellicose language about military force had been removed.

The third point she left for her campaign staff to make. As Clinton’s Iowa spokesman Mark Daley said, "If Senator Obama really thinks this is an issue of war and peace, it's odd that he didn't speak out against the measure before it passed or show up to vote against it."

Clinton’s campaign made all those points in a mass mailing to Iowa voters last weekend that featured Durbin and Clinton supporter Gen. Wesley Clark. That was countered by an Obama mailing just days later, calling the amendment "dangerous."

"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," reads the Obama handout.

But after the Craig memo on Thursday, Clinton’s camp went on offensive again with its own memo. Pointing to comments Obama himself made in 2006 arguing for maintaining troops in Iraq in order to keep Iran in check, the unsigned note rips the Illinois senator for playing politics and lacking true leadership.

"Stagnant in the polls and struggling to revive his once-buoyant campaign, Senator Obama has abandoned the politics of hope and embarked on a journey in search of a campaign issue to use against Senator Clinton," it reads. "Never mind that he made the very argument he is now criticizing back in November 2006."

While the Clinton camp explains its response by saying it refuses to let unfair attacks go unanswered, the rare broadside against Obama can be interpreted as a sign of concern that at least some of the shots from his campaign are landing in Iowa.

With his constant attacks on the issue, Obama in effect has placed at least part of the responsibility for any U.S. action against Iran before the caucuses on the shoulders of Clinton.

But Obama isn’t exactly in an enviable position either. The famed judgment that led him to oppose the measure leading to war in Iraq in 2002 apparently deserted him before the Kyl-Lieberman amendment. The Clinton campaign hopes Obama's failure speak out or show up to vote against an amendment he now says is crucial — a point he conveniently leaves out when he says he opposed the resolution — comes off as pure political expediency.

The latest memoranda war make it clear that the battle isn’t over — and it’s not just limited to Clinton and Obama. John Edwards got into the act on Friday, saying Clinton "aided and abetted George Bush and Dick Cheney’s march to war." And Joe Biden criticized both frontrunners for their actions, saying "Unlike Senator Clinton, I don’t trust this administration to follow the plain meaning of the law. And unlike Senator Obama, I believe this was a vitally important vote — not one to miss and then complain about later."