WASHINGTON - U.S. Sen. Ron Johnson of Wisconsin cast a key vote Tuesday in favor of beginning debate on a Republican rewrite of the Affordable Care Act.

Johnson, who more than a month ago floated the possibility that he might oppose a motion to proceed on a health care bill, gave Republican senators the 50 votes they needed to formally begin debate on the issue. Vice President Mike Pence cast the 51st and tie-breaking vote.

"If we didn't proceed to a bill, the process would have ended and the mess that is Obamacare would have remained in place," Johnson told reporters after the vote. "We've got to keep the process going."

Meanwhile, Sen. Tammy Baldwin of Wisconsin and Sens. Al Franken and Amy Klobuchar of Minnesota joined with all other Democrats and two Republicans in opposition.

"Let's be very clear about what just happened: In defiance of a majority of Americans, Senate Republicans have set in motion a dangerous and destructive process that could result in millions of people losing their insurance coverage, the destruction of Medicaid as we know it, and the elimination of nationwide protections for people with pre-existing conditions," Franken said after the vote.

While the Senate's vote Tuesday does not necessarily mean a health care bill will pass, "no" votes by three Republican senators would have effectively killed the plan.

It was against that backdrop that Johnson cast his "yes" vote in dramatic fashion. Johnson and Arizona Sen. John McCain were the last two Republicans to vote on the motion - and by that time, two other Republican senators, Susan Collins of Maine and Lisa Murkowski of Alaska, already had voted against it.

With the outcome of the plan hanging in the balance, Johnson spent several minutes speaking face-to-face with Republican Senate Majority Leader Mitch McConnell of Kentucky, as the two stood over a desk in the middle of the Senate chamber.

"I just wanted to convey to him that I wanted to continue to be a positive influence on trying to get as good a bill as possible," Johnson said.

McCain, who was just diagnosed with brain cancer, was greeted with applause as he arrived in the Senate chamber. He voted yes to proceed and Johnson quickly did the same.

Democrats blasted Johnson, pointing out that as recently as Monday, he'd said he "didn't have a clue" what the Senate would be voting on.

"Sen. Ron Johnson isn't listening to the people he represents by voting to advance the repeal of the Affordable Care Act and strip health care insurance away from hard-working Wisconsin families," said Democratic Party of Wisconsin spokesman Brandon Weathersby.

How the process unfolds from here and what the Senate votes on next remains to be seen. While more moderate senators have expressed concern over the number of people who could lose insurance under earlier Republican bills, Johnson and other conservatives have complained the plans don't address health care costs.

Johnson said he wants to make sure the final bill addresses the growing cost of Medicaid and provides parity between states that expanded the program under Obamacare and states such as Wisconsin that did not.

"I would support voting for full repeal (of the Affordable Care Act) in a heartbeat, but only to set up full replacement," Johnson said.

Wisconsin Public Radio can be heard in the Twin Ports at 91.3 FM or online at wpr.org/news. Don Davis of Forum News Service contributed to this report.

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Find more coverage of Tuesday's health care vote in the Senate on Page A8