Patrick, during his monthly "Ask the Governor" program on WGBH-FM, also said that his differences with legislative leaders over the bill were about policy and did not reflect any personal feud.
The governor had previously said he would not sign the bill, which was sent to his desk last month, unless the amendment was added. But he had not previously used the word veto, a nuance that left open the possibility that he could let the measure become law without his signature.
Complicating the dispute is a Friday deadline for Patrick to act on a $34 billion state budget, which is balanced in part by new taxes contained in the unsigned bill that is aimed at solving the state's chronic transportation funding woes.
Patrick contends that the bill - which promises $800 million in new state revenues dedicated to transportation by 2018 - does not take into account the loss of about $135 million a year if the tolls west of Interstate 95 come down as scheduled in 2017. The amendment would allow the gas tax to increase to offset the loss of the tolls.
Senate President Therese Murray and House Speaker Robert DeLeo oppose the amendment and are urging members to reject it when it comes up for a vote next week. The bill passed both chambers by margins that, if unchanged, would be wide enough to override a veto.
Patrick, who originally sought $1.2 billion in new revenue for transportation, has repeatedly expressed frustration with the Legislature's handling of the measure.
"The senate president and the speaker can do whatever they want down the hall," he told reporters Wednesday. "What the public needs to know and what their members need to appreciate is that this $800 million bill is not in fact an $800 million bill."
The governor has also been critical of legislators for not holding a public hearing before voting on their version of the transportation bill. Patrick, DeLeo and Murray have also skipped several of their usual Monday afternoon meetings in recent weeks, leading to speculation that relations with legislative leaders had grown chilly as Patrick entered his final 18 months in office.
"This is a policy difference. It's not a personal one," the governor insisted on Thursday's radio program. He added that the leaders appeared to be "dug in" on their position.
The bill contains $500 million in new taxes that are needed to balance the state budget for the fiscal year that started July 1. The governor said he was weighing his options before Friday's budget deadline, but one possibility would be to use his line-item veto power to trim the budget to reflect the absence - for now at least - of the new taxes.
The state is currently operating on a stopgap budget that runs through the end of July.