Michigan Republican Gov. Rick Snyder's plan to expand Medicaid to more poor residents under President Barack Obama 's health care reform law ran into a roadblock it may not be able to surmount on Thursday.

The Michigan State Senate's Republican leadership declined to allow a vote on a plan to add about 450,000 state residents to Medicaid under a House-passed proposal pushed by Snyder. Snyder is one of nine Republican governors to break with their party and support this core element of Obamacare. Republican legislators in Maine and New Hampshire also have blocked Medicaid expansion efforts since Wednesday.

As state legislatures across the country have been winding down their sessions, it appears that only about half of the states will adopt the Medicaid expansion, meaning millions of low-income people will be denied health benefits. According to the health care consulting company Avalere Health, about 3.4 million fewer people will gain health coverage under Obamacare because of states opting out of the Medicaid expansion.

The health care reform law calls for expanding Medicaid , the joint federal-state health program for the poor, to anyone who earns up to 133 percent of the poverty level, which is $15,282 for a single person in 2013, starting next year. But the Supreme Court ruled last year that states didn't have to participate.

The setback in Michigan -- where the issue could reemerge when the legislature returns in late August -- followed the Wednesday failure of the Democratic majority in Maine's legislature to override a veto of their Medicaid expansion legislation by Gov. Paul LePage (R). It also comes after the decision early Thursday morning by a key committee in New Hampshire's divided legislature to postpone a decision on expansion until August.

"There is an opportunity here to provide coverage to a lot of people," Snyder said during a news conference in Lansing. "It's an opportunity to help people in need"

Snyder, who endorsed the Medicaid expansion in February , has said that he opposes Obama's health care law, called the Affordable Care Act, but thinks the Medicaid expansion should go forward in Michigan to extend coverage to the working poor, ease the burden on hospitals that treat patients who can't pay and ensure that residents' federal taxes return to the state.

"There are major negatives to the Affordable Care Act, but there are some positives. It's a mixed bag. The question is: What do you do when you have to recognize it's the law of the land?" Snyder said.

Snyder heavily lobbied Republican state senators on the Medicaid bill, even returning from an official visit to Israel early so he could meet with lawmakers Thursday. But state Senate Republican leaders resisted holding a vote because at least half of the caucus didn't support expanding Medicaid.

"I want to make a call to all Michiganders to join me in speaking to the Senate Republicans to say, 'Take a vote, not a vacation,'" Snyder said.

Snyder predicted enough Democratic and Republican senators would have joined together to pass the so-called Healthy Michigan legislation, which passed the state House last week.

"I believe a majority of Michiganders support Healthy Michigan. A majority of the House of Representatives support Healthy Michigan. I, as governor, support Healthy Michigan. I believe there's actually most likely a majority in the Senate that supports Healthy Michigan. But if it doesn't come to a vote, it doesn't get done," he said.

The state Senate could revisit the issue when it resumes legislating at the end of the summer. Snyder said the issue was more urgent, in part because of the administrative challenges of getting an expansion ready by the beginning of next year, when it would take effect. Snyder said he hasn't decided whether to call the legislature back into session or take other steps to force a vote.

"We're out of time. We need to move forward," Snyder said. "Not making a decision is making a decision -- a decision of no."

Twenty-three states and the District of Columbia are moving forward with the Medicaid expansion, according to an analysis by the Henry J. Kaiser Family Foundation. Most Republican governors oppose the expansion as part of Obamacare and as a potential drain on state coffers.

Several GOP governors, including Ohio's John Kasich and Florida's Rick Scott, backed the expansion but have been stymied by Republican-led legislatures . By contrast, Arizona Gov. Jan Brewer (R) strong-armed a Medicaid expansion through the GOP-led legislature this month, and Iowa Gov. Terry Branstad (R) struck a deal on Medicaid with GOP lawmakers in his state in May. In Arkansas, Democratic Gov. Mike Beebe also forged a compromise plan with the Republican-led legislature.

Snyder's plan would have increased the role of private health insurance companies in covering low-income people eligible for Medicaid, similar to the laws enacted in Arkansas and Iowa.

Under Obama's health care law, the federal government will cover 100 percent of the costs for newly eligible people from 2014 through 2016, after which the share will gradually decline until it reaches 90 percent in 2022 and later years. That compares to the average 57 percent the federal government pays for current Medicaid beneficiaries.

According to the Rand Corp., a consulting firm, states that don't expand Medicaid collectively will forgo $8.4 billion in federal funding and face increased state spending on medical care for indigent uninsured people.