I-495 looking north from Tyson's Monday November 19, 2012. (Dayna Smith/FOR THE WASHINGTON POST)

RICHMOND — Gov. Robert F. McDonnell will unveil a plan Tuesday to pump at least $500 million a year into the state’s quickly draining transportation coffers.

So far, the Republican governor has revealed where about half of that money would come from: the state’s general fund.

He has said he wants to spend $48 million in general funds on transportation in 2014, ramping that up to $275 million a year by 2018.

That part of the plan is sure to face opposition in the state Senate, which shot down a similar proposal last year, arguing that it would cheat schools and other programs that the fund bankrolls.

Republicans and Democrats have been dug in on their conflicting transportation funding positions for more than a decade. Most Republicans have wanted to take existing money from the general fund to pay for roads, while most Democrats have wanted to raise new revenue.

As he enters his last year in office, the term-limited McDonnell is aiming to solve a problem that has eluded his recent predecessors. If he or someone else doesn’t, a state with one of the nation’s largest and most congested transportation systems will be out of road funds by 2017.

McDonnell has not yet disclosed where the rest of the money for his plan would come from. He has hinted that he is open to linking the gas tax to inflation, but that would bring in more revenue only if the price of gas rose.

He will present his full plan at a news conference Tuesday afternoon.

What’s known of his plan so far has faced some criticism from all sides.

Anti-tax activist Grover Norquist warned Virginia legislators last month that “indexing” the gas tax to inflation — an idea McDonnell has only floated so far — would violate the no-taxes pledge they’d signed.

At the same time, transportation advocates are contending that $500 million a year will not be nearly enough.

Bob Chase, president of the Northern Virginia Transportation Alliance, thinks the state needs about $1.5 billion a year in new transportation funding: $500 million of it for maintenance and $1 billion for new construction.

Lt. Gov. Bill Bolling (R), a close ally of McDonnell’s who has become more outspoken lately as he considers an independent bid for governor, said in a recent interview that he believes the state needs about $1 billion a year in new road funds.