MADISON, Wis. (AP) — A birthplace of the progressive movement is crackling with a fervor not seen in decades, as students from the famously liberal University of Wisconsin team up with unionized state workers for demonstrations against changes to collective bargaining rights pushed by the state’s new Republican governor. The biggest rally yet was expected Saturday, along with an influx of conservative counter-protesters.

As many as 40,000 people swarmed the Capitol on Friday, raising the noise in its rotunda to earsplitting levels as they rallied to block Republican Gov. Scott Walker’s efforts to ease Wisconsin’s budget woes by cutting many government workers’ pay, benefits and bargaining rights.

No stranger to political unrest, Madison has seen activists take to the streets to protest the Vietnam war, support civil rights and oppose cuts in social services. Riots ensued 15 years ago when police clamped down on an annual block party that began as an anti-war protest in 1969.

Some say last week’s rallies were unmatched in their sustained, impassioned energy — bolstered by Senate Democrats who fled the state to delay action on Walker’s proposal and threatened to stay in hiding for weeks if calls for negotiation go unheeded. State troopers were sent to retrieve the Democratic minority leader from his home Friday, but their knocks went unanswered.

“That’s jaw-dropping. This is uncharted,” said Mordecai Lee, a UW-Milwaukee political scientist and former state lawmaker who said he was reminded last week of when motorcycle riders’ protest of a helmet law in the late 1970s persuaded legislators to overturn the measure.

Democrats who stayed in Madison on Friday scored their own victory, forcing the state Assembly to adjourn until at least Tuesday without taking a vote on Walker’s bill. Republicans, however, have more than enough votes to pass the measure once the Legislature can convene.

The vast majority of the protesters who have for four days filled the Capitol with chanting, drum-beats and anti-Walker slogans have been union workers and their supporters. Tensions could rise Saturday, when conservative counter-protesters were set to arrive by the busload to demand that the bill be passed. Protests are organized by groups including the Tea Party Patriots, the movement’s largest umbrella group, and Americans for Prosperity.

Paul Soglin, who has been at the Capitol all week and spent at least one night on the floor, didn’t seem concerned about clashes with the opposition, saying he’s been struck by protesters’ positive enthusiasm.

“A joy, yes, in the way people greet one another, the way they’re energized by one another,” said Soglin, who described himself as a veteran of more than 100 protests since the 1960s. “They’re excited that even though there’s a grim prospect of the bill being adopted, that in the long run they’re building something that can be strong for the working class.”

Walker insists the concessions he is seeking from public workers — including higher health insurance and pension contributions — are necessary to deal with the state’s projected $3.6 billion budget shortfall and to avoid layoffs. Eliminating their collective bargaining rights, except over wage increases not greater than the Consumer Price Index, is necessary in order to give state and local governments and schools flexibility to deal with upcoming cuts in state aid, Walker said.

Sarah Palin weighed in with a Friday night posting on her Facebook page that urged “union brothers and sisters” not to ask taxpayers to support “unsustainable benefits packages.”

“Real solidarity means everyone being willing to sacrifice and carry our share of the burden,” Palin said in her post, which did not indicate whether she would join conservatives in Madison this weekend.

Dane County Sheriff Dave Mahoney predicted crowds could swell to as many as 70,000 people on Saturday and said his department planned to add 60 deputies to the 100 who patrolled during the week.