In a move that could increase tensions inside Virginia's Republican Party, Attorney General Ken Cuccinelli on Monday backed a transportation plan that rivals one introduced by Gov. Bob McDonnell and now threatens his fellow Republican's legacy.

The plan Cuccinelli endorsed is being presented as a conservative alternative to McDonnell's transportation package, which would eliminate the state gas tax and increase the sales tax and certain fees to raise $3.1 billion for roads, a top priority in vote-rich Northern Virginia.

The alternative plan would replace the current gas tax with a sales tax on gasoline that could rise with inflation and produce more money than the current flat tax of 17.5 cents per gallon. The measure also would eliminate $600 million in fee increases McDonnell proposed.

Senate Transportation Committee Chairman Steve Newman, who introduced McDonnell's package, unveiled the alternative proposal late Monday -- one day before the deadline to pass a transportation plan on to the House.

In bucking McDonnell over transportation, Cuccinelli is likely to win the backing of General Assembly conservatives who have been resisting McDonnell's efforts to raise the sales tax.

Cuccinelli never fully embraced McDonnell's proposal the way he has the conservative alternative, which he insists "has the best chance to get the votes needed to make improvements to Virginia's transportation system."

For weeks, McDonnell has aggressively pushed his own transportation funding overhaul, a $3.1 billion reform package that would be the Republican's crowning legislative accomplishment. He has traveled the state pressuring lawmakers to back it and has rolled out endorsements from dozens of organizations, from local Chambers of Commerce to real estate groups.

McDonnell spokesman Tucker Martin downplayed the alternative road plan, noting that it mirrors McDonnell's plan by shifting a portion of the existing sales tax to roads and factors in a future sales tax on online purchases. Still, Martin said the governor's plan "is the broad funding package our transportation system needs to be sustainable in the years ahead."

The Cuccinelli-backed alternative is likely to raise about $1.2 billion less than McDonnell's proposal, but is more palatable to Republicans who do not want to vote for a sales tax increase in an election year. The split in the party, however, has complicated negotiations over transportation funding, said Sen. Don McEachin, D-Henrico.

"It does present some challenges," McEachin said.

Cuccinelli and McDonnell have not always seen eye to eye. McDonnell backed Lt. Gov. Bill Bolling to replace him and chided Cuccinelli for announcing a surprise bid to run as well.

Last year, Cuccinelli's wife told supporters in an email that the state lacked "firm, principled conservative leaders," a slight aimed at McDonnell.

But McDonnell has also hosted two fundraisers in recent months for Cuccinelli and defended the attorney general for breaking tradition and remaining in his current job while running for governor.