RICHMOND -- Virginia Democrats are threatening to block historic transportation funding reforms unless Gov. Bob McDonnell agrees to expand Medicaid to more than 400,000 low-income residents.

Democrats initially appeared ready to vote for the transportation compromise, but a senior Senate Democrat told The Washington Examiner late Thursday that only five Democrats in the upper chamber now support the current proposal. The compromise would pay for road work and new transit projects by raising the sales tax and replacing the tax on a gallon of gasoline with a 3.5 percent tax on wholesale gas and other fees.

The reversal by Democrats came after McDonnell wrote to lawmakers opposing a budget amendment that would allow Virginia to join the massive expansion of Medicaid called for under President Obama's health care law unless significant changes were made in the federal-state program.

The letter angered Democrats, who felt they had made considerable concessions during negotiations over the transportation package, including dropping their opposition to using general fund revenue for roads.

"The governor's letter on Medicaid has soured the mood of bipartisanship," the senior Democrat said.

Lawmakers continue to work on a series of budget amendments that include whether to increase the state Medicaid rolls. A Senate version of the bill would allow participation in the expansion, but the House is following McDonnell's lead. The federal government picks up the tab on the expansion through 2016, when states start paying 10 percent of the cost.

Republicans lashed out at Democrats for holding transportation hostage to Medicaid.

"The two issues have no business being linked," said House Majority Leader Kirk Cox, R-Colonial Heights.

Democrats used a similar tactic to block the budget last year, holding out for weeks until McDonnell agreed to set aside $300 million for the Metro Silver Line to Washington Dulles International Airport. McDonnell and his fellow Republicans refused, however. The current transportation package includes money for the Silver Line.

"We can't imagine that Democrats would force Virginians to keep sitting in traffic, and thwart a once-in-a-generation transportation fix, because of a completely unrelated issue," McDonnell spokesman Tucker Martin said. "How would they explain that kind of obstruction to their constituents?"

The Obama administration signaled this week that it would allow Virginia to seek the reforms McDonnell called for, but that did not placate McDonnell.

Other Republican governors have recently embraced Medicaid expansion, including Govs. Rick Scott of Florida, John Kasich of Ohio and Jan Brewer of Arizona.

"We are the outliers," said House Minority Leader David Toscano, D-Charlottesville, "and I fear that the political calculations are trumping the economic [benefits of expanding Medicaid]."