The Minnesota Legislature edged Saturday toward a dramatic showdown over the most sweeping union expansion in recent history as legislators braced for an overnight marathon debate on the proposal.

The highly anticipated watershed over whether child-care workers can unionize came after an already grueling day that saw legislators take up several large budget measures and race to finish last-minute changes to a $2 billion tax bill that includes aid for hallmark projects, including the Mayo Clinic and Mall of America. Legislators must adjourn the session by midnight on Monday.

Fiery crowds on both sides of the union debate built in the Capitol throughout the day, some of them having already fought over the issue for close to a decade.

"We'll have a legal voice to collectively bargain with the state," said Sharon O'Boyle, a child-care provider in Washington County. "That's what we need — we need that legal voice."

Jennifer Parrish operates a family child-care business in Rochester and has been fighting child-care unionization for eight years. "I'm tired, and quite frankly, I'm broke," she said. "The way the bill is written, the deck is definitely stacked in the union's favor."

The labor debate is the last stand for opponents of a proposal to allow child-care and home health workers to unionize. If successful, the state's two largest unions could add more than 20,000 new dues-paying members. Those unions could then represent the workers in negotiations with the state. Unions would have until 2017 to convince home health workers or child-care workers to organize. Neither group would have the right to strike, according to the proposal.

Many political watchers consider it the most consequential vote of the session, potentially more destructive to the DFL majorities than last week's vote to legalize same-sex marriage.

"I believe this is more of a private business issue and an attack on entrepreneurship," said Rep. Tim Kelley, R-Red Wing. About 70 percent of messages from his district have been against the measure, he said. "I guess overreaching would be not a strong enough word."

Rep. Jason Metsa, DFL-Virginia, said that unionized child-care providers would be in a stronger position to fight for better funding, and better care, for children.

"It's really about kids when you come down to it, and making sure the funding is there for them," Metsa said. "Giving people the opportunity to vote for themselves is what we are doing here. The Legislature is not creating a union."

The union proposal stirred the Senate to a 17-hour debate last week, breaking records for length and betraying a rancor on an issue that threatens to consume the end of the session. The Senate passed the measure by only three votes.

The House debate, which had not started at the time this edition went to press, is expected to match the Senate's marathon debate and stretch well into Sunday.

Adding to the frenzy inside the Capitol, activists on both sides of the unionization issue were joined by advocates for driver's licenses for undocumented immigrants. A group of traditional Aztec dancers fired up the crowds as they sought to urge the House and Senate to take up the drivers' license bill. Gov. Mark Dayton opposes the bill, which kills its chances for the year.

The unionization debate, which will be largely driven by Republicans, stands to gobble up crucial hours that could jeopardize Democratic leaders' desire to finish before Monday's mandatory adjournment deadline.

DFL legislators now have just two days to pass their tax proposal, the linchpin of their two-year budget. The governor and DFL legislative leaders already have agreed on the two key components — an income tax hike on high earners and a $1.60-per-pack hike in the tobacco tax, which would more than ­double the existing levy.

The crunch leaves lawmakers little time to sort through a blinding array of smaller, but crucial details.

Pay raise for nursing home workers

The Senate on Saturday gave final passage to a massive $11 billion health and human services bill that would cut $50 million, but also give nursing home workers a 5 percent pay raise. The bill, which now goes to Dayton, includes extra funding for in-school mental health care and aid to families seeking expensive autism ­therapy for their children.

A K-12 finance bill was expected to get final approval by both houses sometime late Saturday, with more bills expected to pass Sunday, after the union debate was exhausted.

On Friday night, a tax committee patched together final details of the state's assistance to Mayo ­Clinic's proposed multibillion-dollar expansion in Rochester. The complex subsidy appeared dead several times over the last few months, only to be resurrected in recent days.

When the committee gave its unanimous endorsement to the project, a rare wave of applause erupted in a room stuffed with groggy lobbyists, legislative staffers and tax watchers. House Taxes Chairwoman Ann Lenczewski, a relentless critic of the earlier funding plan, apologized "for being such a pest" throughout ­negotiations, drawing ­laughter.

GOP Sen. Dave Senjem, a retired manager at Mayo and a lead supporter of the project, gave a long hug to Rep. Kim Norton, a Rochester DFLer who also pushed the plan.

Tears streamed down ­Senjem's face as he returned to his chair.

Legislators showed the toll of grueling hours and long floor debates by Saturday afternoon when there were flashes of snippiness as clearly doomed proposals dragged out the debate.

"We still have to housebreak you of some of your behavior," Senate Majority Leader Tom Bakk said, scolding freshman Sen. Torrey Westrom, R-Elbow Lake, after Westrom made a proposal he later withdrew.

Stakes high as time runs out

The high-stakes logistics of these final few days stand as the biggest test yet of the planning and resolve of the new Democratic leaders. With so little time left and so many spending measures to slog through, the session could implode and force legislators into a special session. That would be a political embarrassment at a time of one-party control, and an additional taxpayer expense.

The frantic flurry of bills also crystallized how Republicans, squarely in the minority in both the House and Senate, are seldom able to block any major initiatives. Dayton and DFL legislative leaders negotiated the entire budget on their own.

House Republicans might not be able to block the unionization proposal, but House Minority Leader Kurt Daudt, R-Crown, said GOP members plan to make sure every one of their arguments are aired through more than 100 amendments to the unionization bill.

Republican opponents call the bill a prime example of liberal DFL "overreach" and a payback to two unions that contributed upward of $2 million to DFL candidates and causes in 2012 election, helping them lock up legislative control.

Supporters say the bill does not require unionization, but merely gives workers the right to vote. Opponents promise to take their battle to court if the bill passes.

Staff writers Jennifer Brooks and Rachel E. Stassen-Berger contributed to this report. • 651-925-5044 • 651-925-5042