Gov. Dannel Malloy has made transportation central to his second-term agenda. At the core of his vision for Connecticut is an ambitious $100-billion transportation “lockbox,” which would contain funds exclusively for projects to refurbish Connecticut’s decaying infrastructure.

Spending money is one thing, though. Raising it is quite another.

The latter issue has vexed state government this summer. Malloy’s plans for a study to investigate the possible effects of a “mileage tax” — which would levy a certain fee per mile driven, as opposed to the state’s current tax on gas and oil — have stirred controversy in the halls of the General Assembly. Legislators on both sides of the aisle have united in opposition to the tax, with Senate Majority Leader Bob Duff, D-Norwalk, declaring it “dead” in July.

The summer’s drama over the mileage tax — just the latest iteration in a long-running debate over revenue sources from drivers — began in late June, when the Washington Post reported that Connecticut, along with a consortium of other northeastern states, was seeking federal funds for a pilot program. It later emerged that Connecticut would supply matching funds of roughly $300,000 for the study, as would each of the states taking part.

Malloy has framed the issue as one of mere fiscal prudence. As cars become more fuel-efficient and as gas becomes cheaper, revenues from the gas tax will fall, leaving the state’s coffers lacking the funds they desperately need.

But reactions in the General Assembly have been markedly negative, with legislative leaders from both parties declaring the tax dead before it even arrived on their desks. Duff, a powerful figure in the state Senate, has been one of the fiercest, and most colorful, critics of the tax, which has pitted Malloy against legislators from his own party — and not for the first time this year.

“The Department of Transportation can study it nine ways to Sunday, up, down, sideways and diagonally,” Duff said in a July statement. “It is an unproven idea and the Senate Democrats will not advance any future proposal to consider a mileage tax for a vote.”

Duff also took a shot at his adversary, Senate Minority Leader Len Fasano ’81, R-North Haven, whom he accused of using the mileage tax as a tool to stir up angry passions among Republican voters, describing Fasano’s rhetoric as a “fear mongering Trump-like tactic.”

Sen. Toni Boucher, R-Wilton, has also taken a strong stance against the mileage tax, deeming it “unwaveringly unpopular.”

“If you thought the idea of tolls was unpopular, just wait until you try to tax Connecticut residents for every single mile they drive,” Boucher said in a June statement. “In the places where the mileage tax has been introduced, it has been highly unpopular and has received a stinging response from the public.”

Fasano, for his part, has maintained that Malloy’s repeated dodging of questions regarding the mileage tax is part of a concerted strategy to mislead voters of the governor’s true intentions and has demanded that Malloy come down firmly on one side of the debate.

Malloy’s true intentions, Fasano has intimated, are to introduce a mileage tax through the back door once it is politically convenient to do so.

“I think there’s the full intention of this governor to put in the vehicle mileage tax,” Fasano told reporters earlier this summer. “They don’t want to announce it until after the November elections, and that’s what this game is.”

In July, Fasano sent a letter to the Democratic leaders of the General Assembly formally requesting a special session, partly for the purpose of passing legislation to prohibit the mileage tax from ever becoming reality. His request for a special session never came to fruition, but the tax’s political fortunes look moribund anyway.

Connecticut is not the only state where proposals for a mileage tax have floated through the legislature. In Massachusetts, Gov. Charlie Baker, a Republican, vetoed a mileage tax pilot program similar to the one proposed in Connecticut, noting that it might disproportionately hit drivers in the rural western half of the state.