Kennedy's legislation comes just one day before the president reveals what he's calling a new way forward in Iraq — a plan that is expected to include the deployment of 20,000 additional troops.
Kennedy, a long-time critic of the president and the war, said sending more troops will not solve the mounting sectarian violence in Iraq.
"The legislation that we will introduce today is brief but essential," Kennedy said in a speech to the National Press Club in Washington. "It requires the president to obtain approval from Congress before he sends even more American soldiers to Iraq. And it prohibits the president from spending taxpayer dollars on such an escalation unless Congress approves it."
"The mission of our armed forces today in Iraq bears no resemblance whatever to the mission authorized by Congress," Kennedy said. "President Bush should not be permitted to escalate the war further and send an even larger number of our troops into harm's way, without a clear and specific new authorization from Congress."
Kennedy called any "surge" of U.S. troops to Iraq an "immense new mistake."
"An escalation, whether it is called a surge or any other name, is still an escalation, and I believe it would be an immense new mistake. It would compound the original misguided decision to invade Iraq," he said.
Like the war in Vietnam, Kennedy said the war in Iraq cannot be won militarily.
"Iraq is George Bush's Vietnam. As with Vietnam, the only rational solution to the crisis is political, not military. Injecting more troops into a civil war is not the answer," Kennedy said.
As the president's speech drew near, White House press secretary Tony Snow insisted that the president was still listening to ideas from lawmakers.
"I'm not saying that the president's going to go back in and shred it and start over," Snow said. "What I'm saying is the president still continues to have an open mind, because this is a way forward. This is not, 'Wave a wand and it's going to happen.'"
Snow conceded that Mr. Bush has a challenge in persuading a war-weary public to send additional troops to Iraq.
"The president will not shape policy according to public opinion, but he does understand that it's important to bring the public back to this war, and restore public confidence and support for the mission," Snow said.
Democrats seem divided on whether to block funds for troop increases, but many were not ruling it out. Senate Majority Leader Harry Reid, D-Nev., said Democrats would "look at everything" in their power to curb the war, short of cutting money for troops already in the field.
He said he would only consider an increase in U.S. forces in Iraq if Mr. Bush agreed to start withdrawing troops within six months.
If brought to the floor by Democratic leaders, Kennedy's proposal would force Republicans to put themselves on record regarding the war for the first time since the Nov. 7 elections, when the GOP lost control of Congress to the Democrats in large part because of the war. Most Republicans say they back the president, or are at least willing to hear him out, but a few GOP moderates say there is no indication U.S. troops would make a difference.
According to senators who attended a meeting Monday with the president, a promise to send more troops to Iraq would be conditioned on criteria met by the Iraqi government, such as reaching political deals on sharing the nation's oil resources and dispatching more of its own troops to Baghdad.
Mr. Bush told the senators that Iraqi Prime Minister Nouri al-Maliki suggested the plan when the two met in late November in Amman, Jordan. The senators said the president expressed confidence that the Iraqi government could meet certain milestones in exchange for additional U.S. support.
But several of the senators remained skeptical.
"We've had these benchmarks before and to no avail," Sen. Olympia Snowe, R-Maine, said after meeting with Bush. "Why should we increase our exposure to risk?"
But whether Snowe and other GOP Senate skeptics of Mr. Bush's plan, including Gordon Smith of Oregon and Susan Collins of Maine, will agree to Kennedy's plan is doubtful.
"It would be a dishonorable thing for the Congress to budget away the bullets at a time when their commander in chief had ordered them to hold their place in the battlefront," said Smith.