LANSING – The amount of tax revenue Michigan can expect to collect over the next three years hasn't changed much since January and will be relatively flat, according to the Legislature's two fiscal agencies.

So if Gov. Gretchen Whitmer wants about $2 billion a year more to "fix the damn roads," that extra money is not going to be generated by economic growth, according to recent reports filed by the Senate Fiscal Agency and the House Fiscal agency.

Those two reports — plus a third revenue projection from the Treasury Department — will be key documents when lawmakers and state financial officials meet at the Capitol on Friday to reach a consensus on how much tax money the state can expect to collect in 2019, 2020 and 2021.

The revenue estimating conference — held twice each year, in January and May — is often a crucial step in finalizing the state budget. That's because changing economic outlooks and activities sometimes result in swings of several hundreds of millions of dollars — or more — in Michigan's revenue picture from one conference to the next.

But no massive swings are expected this year.

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Both agencies project slowing growth — below that of the U.S. as a whole — and below the projected inflation rate of about 2%.

The Senate Fiscal Agency, in a report released Thursday, said combined general fund and School Aid Fund net revenues will be about $111 million higher in 2019 than the January projections showed. For 2020, net revenues will be down about $203 million from the January projections and for 2021, down about $178 million, the report said.

Those adjustments are relatively small, when discussing more than $24 billion in total revenues.

The report from the House Fiscal Agency, while more optimistic over the entire forecast period, shows even less variation from the January estimates, with projections edging up by about $50 million or less for each of the three years.

The revenue conference is held Friday as the Republican-controlled Legislature continues to advance budget bills that vary significantly from what Whitmer, a Democrat, recommended in March.

The biggest omission is Whitmer's proposed 45-cent gas tax increase, which would raise $2.5 billion gross, and $1.9 billion net to fix Michigan roads and bridges. Leaving the gas tax out of the budget has implications for education and environmental spending, since Whitmer wants to direct general fund dollars that are currently targeted for roads toward those priorities.

Although budget deliberations are at an advanced stage, no Democratic lawmaker has yet introduced a bill to provide for the 45-cent gas tax hike, which polls show is not a popular idea.

Republican lawmakers have pointed to the lack of a gas tax bill as one reason they can't build that revenue into the budget.

"I think we have to move forward with a real budget with real revenue,' said Sen. Jim Stamas, R-Midland, chairman of the Senate Appropriations Committee. "And that is exactly what the Senate has proposed.”

House Minority Leader Christine Greig, D-Farmington Hills, said the shape the budget takes will be a product of negotiations with the Whitmer administration and others.

At this point, there is no reason to introduce a gas tax bill, because Republicans "would use it as a political tool," perhaps by forcing a quick vote on the issue, Greig said.

Contact Paul Egan: 517-372-8660 or pegan@freepress.com. Follow him on Twitter @paulegan4. Read more on Michigan politics and sign up for our elections newsletter. The Associated Press contributed to this report.