MADISON, Wis. — 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 is 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 this week’s rallies are 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’s been reminded this 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 are 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.