"The president does not have the authority to launch military action in Iran without first seeking congressional authorization," Senate Democratic leader Harry Reid, D-Nev., told the National Press Club.
The administration has accused Iran of meddling in Iraqi affairs and contributing technology and bomb-making materials for insurgents to use against U.S. and Iraqi security forces.
President Bush said last week the U.S. will "seek out and destroy" networks providing that support. While top administration officials have said they have no plans to attack Iran itself, they have declined to rule it out.
This week, the administration sent another aircraft carrier to the Persian Gulf — the second to deploy in the region. Defense Secretary Robert Gates said the buildup was intended to impress on Iran that the four-year war in Iraq has not made America vulnerable. The U.S. is also deploying anti-missile Patriot missiles in the region.
The U.S. has accused Tehran of trying to develop nuclear weapons. Iranian President Mahmoud Ahmadinejad said Thursday that Iran would not back down over its nuclear program, which Tehran says is being developed only to produce energy.
Reid made the comments as he and House Speaker Nancy Pelosi, D-Calif., spoke to the National Press Club on Democrats' view of the state of the union four days before Bush addresses Congress and the nation. His remarks were the latest Democratic display of concern about the possibility of military action in Iran and Bush's power to launch it.
Last week, Senate Foreign Relations Committee Chairman Joseph Biden, D-Del., challenged the president's ability to make such a move. In a letter to Bush, Biden asked the president to explain whether the administration believes it could attack Iran or Syria "without the authorization of Congress, which does not now exist."
Meanwhile, Lee Hamilton, the Democratic co-chair of the Iraq Study Group, told the House Foreign Affairs Committee on Friday that the U.S. must try to engage Iran and Syria in a constructive dialogue on Iraq because of the countries' influence in the conflict.
The Bush administration, and several members of Congress, say they oppose talks with Iran and Syria because of their terrorist connections. Bringing the two countries into regional talks aimed at reducing violence in Iraq was one of the study group's recommendations.
"Do we have so little confidence in the diplomats of the United States that we're not willing to let them talk with somebody we disagree with?" Hamilton asked.