Democratic lawmakers in Wisconsin have left the capitol to try to slow a bill aimed at curtailing public employees' collective bargaining rights.
Wisconsin teachers, prison guards and others crowded into the capitol to protest against the Republican bill.
They described it as an attack on workers' livelihoods.
Republicans in Washington DC and state capitals have moved to cut government spending this year, including on public worker pay, in a bid to curb deficits.
The legislation had been expected to pass the Republican-led Wisconsin state legislature on Thursday.
But Wisconsin Senate rules require 20 senators to be present for a quorum; Republicans hold 19 seats and the Democrats 14. Senate Democrats did not show up for the session, and aides told reporters they did not know where the legislators had gone.
The Democratic senators said they would not return before Saturday.
Democratic Senator Jon Erpenbach said Democrats had left to slow down the bill in the hope of forcing Republican Governor Scott Walker and Republican legislators to negotiate.
"What we're trying to do is get the governor to sit down and at least try to talk with people who have some issues with what he's trying to do," said Mr Erpenbach told a Wisconsin radio station over the telephone from Chicago.
"This isn't about the money. This is all about the collective bargaining rights that the governor wants to take away from the unions."
Mr Walker, meanwhile, called on the Democrats to return.
"The state senators who are hiding out down in Illinois should show up for work, have their say, have their vote, add their amendments," he told CBS news on Friday, "but in the end, we've got a $3.6bn (£2.23bn) budget deficit we've got to balance."
President Barack Obama, meanwhile, stepped into the issue, describing the bill as "an assault on unions" during an interview with a Milwaukee, Wisconsin television station.
The union workers and their supporters cheered the move.
"The fact that the Democrats have walked out, it shows they're listening to us," Neil Graupner, a 19-year-old technical college student told the Associated Press late on Thursday.
In Madison, the capital city of the mid-western state, the legislature on Thursday had been set to pass a bill pushed by the governor that has been described by commentators as the most aggressive anti-union law in the nation.
The bill would eliminate most public workers' right to collective bargaining, except for matters of salary, and dramatically increase the amount they must contribute to their pensions and health insurance coverage.
With teachers - and some students - massing in Madison to protest, dozens of schools were shut across the state.
'Point of crisis'
Republicans, who were handed election victories in November in Wisconsin, say they have a mandate to cut government spending.
They say that despite the protests, voters approve of the cuts, which they say are needed to balance the state budget and avoid job losses.
"We're at a point of crisis," Mr Walker said.
The state faces a $3.6bn (£2.23bn) budget deficit in the coming two-year period. The public employee bill is expected to save $300m in that period.