Tens of thousands of demonstrators staged a fourth successive day of protests in Wisconsin against the most sustained "union-bashing" measures proposed in the US for decades.

Wisconsin has become a test-bed for Republicans planning similar anti-labour moves elsewhere as they seek to remove union rights and cut benefits in states facing huge holes in their budgets.

Unions, backed by Barack Obama, have responded by mobilising workers throughout the country in support of Wisconsin workers.

The Republicans provoked the Wisconsin confrontation by trying to pass legislation on Thursday to remove the collective bargaining rights of an estimated 300,000 workers, ranging from teachers to prison guards. The bill was delayed by an odd technicality that involved Democratic senators fleeing the state.

The union movement is a shadow of the power it was for much of the 20th century, and cannot afford to lose further ground.

Obama told a Wisconsin TV station that while he understood the need for cuts, the Republicans were going too far. "Some of what I've heard coming out of Wisconsin, where you're just making it harder for public employees to collectively bargain generally, seems like more of an assault on unions," he said.

Many schools in Wisconsin have been closed as teachers joined the protests. The state's Republican governor, Scott Walker, has threatened to bring in the National Guard if services are disrupted.

Union leaders are pouring resources into Wisconsin but are also planning for battles in Ohio, New Jersey, Pennsylvania, Florida, Indiana, Michigan and other states facing budget crises. They have put aside $30m for the campaigns.

"Plans are being put into place to silence workers, lower their wages, cut their benefits and increase the likelihood that they will suffer injuries and fatalities at work," said Gerald McEntee, president of the American Federation of State, County and Municipal Employees, which has 1.6 million members, said. "It is happening at a breakneck pace."

Republicans in the state senate in Ohio are also proposing legislation to end collective bargaining, a move that prompted thousands to protest on Thursday.

James Gregory, a professor of history specialising in labour studies at the University of Washington, said the consequences could be felt for years to come.

"This is a threat not just to unions but to American democratic institutions. The past century has seen a significant expansion of civil rights, including workplace rights, and democratic institutions, including the principle that employees have the right to negotiate terms of employment and be represented by unions.

" For a state government to take away these rights long after they have been established is probably unprecedented and is deeply troubling."

Gregory added: "I can't think of any previous anti-union legislation that represented the same kind of threat. The Taft-Hartley Act passed in 1947 placed restrictions on how unions could operate. Some states passed so-called Right to Work laws beginning in the 1950s."

The move at state level is mirrored in the battle in Washington over budget cuts. Obama, who is planning relatively modest budget reductions given the size of the US deficit, has vowed to resist deep cuts voted through by Republican members of Congress. The Republican leader in the House, John Boehner, has hinted that the federal government may be closed down from 4 March, the budget deadline.

The Wisconsin legislation would remove workers' rights, in place since a landmark decision in 1959 that is part of union history, to negotiate over pensions and health insurance, both of which are being cut. They will be allowed to continue bargaining over wages. The bill means state workers will have to pay half their pension costs and at least 12.6% of their healthcare coverage.

Police and firefighters were excluded, though firefighters took part in the protests in a show of solidarity.

The legislation also weakens the unions by dictating elections and changing rules on collecting union dues.

Walker said the "modest" cuts in pensions and health insurance would result in savings of almost $30m by July and almost $300m over two years. Wisconsin has a projected $3.6bn budget gap over the next two years.

The legislation was due to be passed on Thursday but was delayed by a technicality. In an almost comic move,14 Democratic senators fled to neighbouring Illinois, where they checked into a modest hotel for the night. Their flight meant the 19 Republican senators were one short of the quorum of 20 needed for passage of the bill. If they had remained in Wisconsin, the police could have rounded them up and forced them to the chamber.

Walker, speaking yesterday on CBS, urged the senators 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, but in the end, we've got a $3.6bn budget deficit we've got to balance," Walker said.

One of the Democratic senators, in an interview with ABC, said: "We left the state so we were out of the reach of the Wisconsin state patrol, which has the authority to round us up and bring us back to the legislature."