A closure would mean the furloughs of hundreds of thousands of workers and the services they provide, from processing many tax refunds to approving business loans. Medical research would be disrupted, national parks would close and most travel visa and passport services would stop, among many others.
Obama spoke after a double-barreled day of meetings with Boehner and Reid. The three have held four such meetings this week.
Throughout Thursday, the president, Reid and Boehner bargained and blustered by turns, struggling to settle their differences while maneuvering to avoid any political blame if they failed.
With the economy just now beginning to create jobs in large numbers, the president said a shutdown would damage the recovery.
"For us to go backwards because Washington couldn't get its act together is just unacceptable," he said.
But agreement remained elusive.
Republicans passed legislation through the House at midday to fund the Pentagon for six months, cut $12 billion in domestic spending and keep the federal bureaucracy humming for an additional week.
"There is absolutely no policy reason for the Senate to not follow the House in taking these responsible steps to support our troops and to keep our government open," Boehner said.
Obama flashed a veto threat even before the bill passed on a 247-181, mostly party-line vote. The administration issued a statement calling it "a distraction from the real work" of agreeing on legislation to cover the six months left in the current fiscal year, and there was no indication Reid would allow a vote on it.
As they left the White House after the evening meeting, Reid and Boehner issued a brief written statement that said they had narrowed their disagreements and said they would "continue to work through the night to attempt to resolve" the remaining ones.
On Friday morning, House Minority Whip Steny Hoyer, D-Md., said that talks have progressed "70 percent of the way on the numbers," but said the two sides are still squabbling about Republican riders to the legislation that would change abortion and environmental policy.
"There's no deal yet," he said on NBC's "Today" show. Hoyer blamed Republicans, saying that "when we were in charge of the House and had disagreements with George Bush, we compromised." He said he's "embarrassed" that Congress has put the country on the brink of shutdown, calling it "inappropriate."
Republicans want deeper spending cuts than the Democrats favor and also are pressing for provisions to cut off federal funds to Planned Parenthood and stop the Environmental Protection Agency from issuing numerous anti-pollution regulations.
"They're difficult issues. They're important to both sides and so I'm not yet prepared to express wild optimism," the president said.
For all the brinksmanship — and the promise of more in the Senate on Friday — there was agreement that a shutdown posed risks to an economy still recovering from the worst recession in decades.
The political fallout was less predictable, especially with control of government divided and dozens of new tea party-backed Republicans part of a new GOP majority in the House. Twin government shutdowns in the mid-1990s damaged Republicans, then new to power in Congress, and helped President Bill Clinton win re-election in 1996.
This time, individual lawmakers worked to insulate themselves from any political damage. Sens. Sherrod Brown, D-Ohio, and Ben Nelson, D-Neb., both seeking new terms in 2012, became the latest to announce they would not accept their congressional salary during any shutdown. "If retroactive pay is later approved, I'll direct my part to the U.S. Treasury," Nelson said. Some two dozen senators of both parties scurried to make similar pledges.
One day before the shutdown deadline, events unfolded in rapid succession.