A coalition including six House members, several U.S. soldiers and parents of servicemen claims only Congress has that power under the Constitution.
"We have a message for President Bush today. Read the Constitution," John Bonifaz, the plaintiffs' lead attorney, said at a news conference announcing the suit.
"A war against Iraq without a congressional declaration of war will be illegal and unconstitutional," he said. "It is time for the courts to intervene."
The representatives joining the suit, all Democrats, are John Conyers of Michigan, Dennis Kucinich of Ohio, James McDermott of Washington, Jose Serrano of New York, Sheila Jackson Lee of Texas and Jesse Jackson Jr. of Illinois.
The U.S. Attorney's office said it had no comment on the suit.
The plaintiffs asked for a preliminary injunction against the president and for a hearing on their request that Bush be barred from launching a military invasion against Iraq without a congressional declaration of war.
The lawsuit cites Article I, Section 8 of the Constitution, which reads: "Congress shall have power... (to) declare war."
The suit argues the resolution on Iraq that Congress passed in October did not declare war and unlawfully ceded the decision to Bush.
The suit contends the framers of the Constitution sought to ensure that U.S. presidents would not have the power of European monarchs of the past to wage war.
"The Founding Fathers did not establish an imperial presidency with war-making power," Conyers said. "The Constitution clearly reserves that for Congress."
"The president is not a king," said Charles Richardson, a plaintiff whose Marine son is stationed in the Persian Gulf.
"If he wants to launch a military invasion against Iraq, he must first seek a declaration of war from the United States Congress. Our Constitution demands nothing less," Richardson said.
Richardson and two other plaintiffs -- Nancy Lessin and Jeffrey McKenzie -- are co-founders of Military Families Speak Out, an organization of people opposed to war against Iraq and who have family in the military.
"A full and complete congressional discussion of the issues and all options must precede any move towards war," Lessin said, "because of the irreparable harm that would result."
At the news conference, Lessin said she worried about her son, Joe, a Marine stationed in the Gulf.
"We worry about Joe," she said. "We don't want him to be wounded or die. We don't want him to be forced to wound or kill innocent Iraqi civilians. That would kill part of him and part of us."