Adding a major new dimension to what's turning into a full-fledged tax fight with the Legislature, Gov. Deval Patrick said Thursday he would veto a $500 million tax plan unveiled earlier this week by legislative leaders.

Patrick is seeking new taxes to pay for $1 billion in transportation system investments, but House Speaker Robert DeLeo and Senate President Therese Murray this week said their $500 million plan is more considerate to taxpayers given the current state of the economy and addresses the state's most pressing transportation needs.

Asked if he would veto the bill, Patrick said at a press conference he called Thursday, "I am saying that if this bill comes to my desk in this form, I am going to have to."

He later added, "I understand that if the legislative leadership wants this bill they will have this bill and they will have my veto." He said he did not know whether he would have the votes to see his veto sustained.

Patrick's more comprehensive tax plan is built on a major income tax hike and a major sales tax reduction and features a long menu of other tax code changes that lawmakers have shown little interest in advancing. The DeLeo-Murray plan leans on cigarette and tobacco, gasoline and business taxes to pull in revenue designed to avert near-term MBTA fare hikes, stop paying for personnel with bond funds, and to shore up regional transit authorities.

Transportation Secretary Richard Davey on Wednesday predicted the DeLeo-Murray plan, if adopted, would force higher transportation fares, fees and tolls and could require the state to pick up the full cost of the $1.3 billion Green Line extension, a project state officials hope will be funded with a 50-50 state/federal funding split.

Senate President Therese Murray on Thursday disputed the administration's claims about higher fares.

"Nope, they're not on the table. That's very clear," Murray told the News Service. "I know the spin is being put out there. It's absolutely not true. We made sure that they would not have to hike fares."

Patrick has touted his plan as a once-in-a-generation opportunity to address longstanding funding gaps, but Murray defended her own proposal Thursday. "It is something we can do now. It won't bankrupt people, and it makes a good investment with our future transportation," she said.

While Murray has explicitly asserted a desire by legislative leaders to protect the middle class from the impact of the tax bill, Patrick said Thursday, ""Let's be clear the leadership proposal taxes the middle class."

Calling the DeLeo-Murray plan a "fiscal shell game," Patrick said it would not lead to the type of system improvements that lawmakers could cite as a rationale for tax votes. "The bill at $500 million means everybody everywhere will be asked to pay more and get the same or less," he said.

The House is scheduled to begin debate Monday on a tax bill that critics from the left say will not generate enough revenue and which has drawn harsh criticism from Republicans opposed to taxes they say are unaffordable.

With the veto threat on the table, DeLeo and Murray will need to decide whether to stick with their plan or whether modify it, with each move potentially building support for the bill or causing more lawmakers to oppose it. It takes a majority vote to pass the bill but a two thirds vote in each branch to override a veto.

Another high-stakes showdown between Patrick and legislative leaders in 2010 led to the failure of legislation authorizing casinos in Massachusetts. Patrick and lawmakers in late 2011 found common ground on casinos and passed a legalization bill, although state regulators are still far from awarding any casino licenses.

Throughout his tenure as governor, Patrick and legislative leaders have been unable to agree on a plan to infuse the transportation system with a major influx of new dollars, which system supporters say is needed to address crumbling infrastructure, antiquated trains and to spur rail service expansion and economic growth.

Patrick does not plan to seek reelection next year, a fact not lost on lawmakers who through history have proven more likely to work with governors who have no plans to move on than with lame-duck chief executives.

House members have until 5 p.m. Friday to file amendments to the proposal. The House on Wednesday rejected a call from Republicans for a public hearing on their bill. Patrick on Thursday also called for a hearing on the bill.