The Michigan Senate is weighing whether to expand Medicaid eligibility to low-income families, a closely-watched vote that has divided conservative Republicans and centrist Gov. Rick Snyder (R) for months.

Copy of 1270177_image_982w Gov. Rick Snyder (Scott Eells/Bloomberg)

The Senate will vote on three different proposals on Tuesday. The version most likely to pass, initially sponsored by state Rep. Matt Lori (R), has already made it through the state House.

Snyder strongly supports accepting federal money provided by the Affordable Care Act to expand Medicaid to families living at or below 138 percent of the poverty level. In an op-ed published Monday, Snyder said the expansion would cover 470,000 Michigan residents.

But conservative Republicans have opposed the bill, which they see as a capitulation to Obamacare that will eventually leave the state on the hook for millions in medical costs. Republicans in the legislature initially rejected Snyder’s request to expand the program when they voted on the budget in March.

“For us, it’s increasing the size of government by another billion and a half,” said Patrick Colbeck, a Republican state senator leading the charge against the House bill and the author of his own alternative. “Medicaid expansion is a key tenet of Obamacare, and it’s the path to single-payer, and you’re not going to see any competition in a single-payer environment.”

Lori’s bill passed the House in June with the support of just 28 Republicans, along with the entire Democratic caucus. House Republican leadership backed the bill, but 31 Republicans voted against final passage. In the Senate, Snyder will need to add at least eight Republicans to the 12 members of the Democratic caucus to win passage. Sources with knowledge of the vote count said exactly eight Republicans have committed to voting for the expansion.

“I’m optimistic they’ll get it through tomorrow,” Lori said on Monday. “People just need to take some time, and not just a couple of hours, need to take a considerable amount of time to study the issue. In the long run, it’ll be the right thing for the citizens of Michigan.”

But in interviews, observers said the final tally was still up in the air as of Monday afternoon. Gretchen Whitmer, the Senate Democratic leader, said her caucus was likely to be unanimously in favor of the bill, barring last-minute changes.

“It may be a close vote. It depends on who you talk to and what time of day you talk to them,” Whitmer said. “We haven’t seen the final version, so it’s hard to say we’re hard-yes votes, but I anticipate we’ll be a unified caucus.”

In the event of a tie vote, Lt. Gov. Brian Calley (R) would be called upon to cast a tie-breaker. Whitmer said that prospect is a distinct possibility.

“It’s going to be a case of ‘Keep your hands and feet in the ride at all times,'” Republican Colbeck said.

Michigan would join 24 other states that are in the process of expanding Medicaid, and only the second state in which Republicans control both the legislature and the governorship. Earlier this month, Arizona Gov. Jan Brewer (R) signed a bill passed through the Republican-controlled legislature that authorized an expansion.

Indiana, Ohio, Pennsylvania, New Hampshire and Tennessee are still debating whether to proceed with Medicaid expansion, according to a tally kept by the Kaiser Family Foundation.

Passing the expansion would be a big win for Snyder, who has struggled with his Republican base in recent months. Polls suggest Snyder’s approval rating is well below 50 percent; a July 23-24 poll conducted by Denno Research for Lambert, Edwards & Associates, a Michigan public relations firm, showed Snyder’s approval rating at just 44 percent. Snyder will likely face former Rep. Mark Schauer (D) in next fall’s elections.