"Congress can prevent it from happening, if they act now," he said in his weekly Saturday radio and internet address.
Following a White House meeting Friday among Obama and congressional leaders, aides to Senate Republican leader Mitch McConnell of Kentucky and Senate Majority Leader Harry Reid (D-Nev.) began racing against the clock for a bipartisan bargain. The leaders could present legislation to senators as early as Sunday, with a vote possible on Sunday or Monday.
The guest list for the White House meeting included Reid, McConnell, House Speaker John Boehner (R-Ohio) and House Democratic leader Nancy Pelosi (D-Calif.). But the key players were clearly Reid and McConnell, both of whom stayed behind briefly at the White House and huddled with their staffs and Obama's top legislative aide, Rob Nabors, in the West Wing Cabinet Room just outside the Oval Office.
Neither side expected compromise to be easy. However, McConnell and Reid voiced unexpected optimism that they could work toward a deal that could win support in both their camps.
Warned Reid: "Whatever we come up with is going to be imperfect."
Looking to add pressure on negotiators, Obama said that absent a compromise he expects Reid to put legislation on the floor to prevent tax increases on the middle class and extend unemployment benefits — an implicit challenge to Republicans to dare to vote against what polls show is popular.
Speaking for Republicans in a Saturday radio address, Sen. Roy Blunt of Missouri sought to put the burden of a deal on Obama and Reid. "We still can avoid going over the fiscal cliff if the president and the Democrat-controlled Senate step forward this week and work with Republicans to solve this problem and solve it now," he said.
Whatever manages to pass in the Senate, with its Democratic majority, would then face a second test in the Republican-controlled House.
Boehner, a Republican speaker who has struggled recently with anti-tax rebels inside his own party, said through an aide that he would await the results of the talks between the Senate and White House. A House vote could come as late as Wednesday, the final full day before a new Congress takes office.
Officials said there was a general understanding that any agreement would block scheduled income tax increases for middle-class earners while letting rates rise at upper-income levels.
Obama was sticking to his campaign call for increases above $250,000 in annual income, even though in recent negotiations he said he could accept $400,000.
The two sides also confronted a divide over estate taxes. Obama favors a higher tax than is currently in effect, but one senior Republican, Sen. Jon Kyl of Arizona, said he's "totally dead set" against it. Speaking of fellow GOP lawmakers, he said they harbor more opposition to an increase in the estate tax than to letting taxes on income and investments rise at upper levels.
But the estate tax was more likely to be used as a possible bargaining chip that Democrats could give away in exchange for higher rates for top earners and other Obama priorities.
Obama and Democrats want to prevent the expiration of unemployment benefits for about 2 million long-term jobless men and women, and there is widespread sentiment in both parties to shelter doctors from a 27% cut in Medicare fees.
One Republican who was briefed on the White House meeting said Boehner made it clear he would leave in place spending cuts scheduled to take effect unless alternative savings were included in any compromise to offset them. In previous White House proposals, Obama has suggested finding enough cuts in government spending to put off the steeper cuts for up to six months.
Obama, speaking to reporters following his meeting with the congressional leaders, faulted a system that left crucial decisions to the last minute, a way of governing that he said the public finds "mindboggling."
"Outside of Washington nobody understands how it is that this seems to be a repeat pattern, over and over again," he said.
Still, Obama himself is part of the negotiating process, and his meeting with all four top leaders Friday was the first since Nov. 16. A phone call he placed Wednesday night to McConnell was the first the Republican leader had received from a Democrat on the fiscal talks since Thanksgiving.