Democratic gubernatorial candidate Jay Gonzalez says he will work with the state’s Department of Revenue on a “proposal to raise additional revenue from those who are doing well in this state” — but he says he can’t give specific details to voters until he is elected.

“You can’t have a detailed proposal for how to do this where you understand what the consequences are in terms of revenues and impacts, in terms of which people are impacted, without the department of revenue doing that analysis,” Gonzalez said in an interview with Boston Public Radio Tuesday. “[Voters] will know, based on my campaign, I’m going to come up with a proposal to raise additional revenue from those who are doing well in this state.”

Gonzalez was a longtime supporter of the ‘millionaires tax’ ballot question, a proposal struck down by Massachusetts’ highest court in June. A ‘yes’ vote on the proposed ballot question would have created an additional 4 percent state tax on annual income earners of more than $1 million. That additional revenue would then be put towards education and transportation. The Supreme Judicial Court rejected the ballot question on a legal technicality.

“I supported the millionaire’s tax,” Gonzalez said. “I am going to propose another progressive way to raise the revenue we need to make the critical investments we need to make in our transportation system.”

“We have one of the worst systems in the country right now,” Gonzalez continued. “People are stuck in traffic for longer, they can’t depend on the T to get to work on time ... we have to be aggressive about this, and we have to be honest about the fact that we need to invest more.”

The best way to make that investment, according to Gonzalez, would be a graduated income tax — but that would mean amending the Massachusetts constitution.

Gonzalez suggested increasing income tax and decreasing deductions and exemptions, but could not say specifically how he would implement such a plan.

“Another way is looking at all of the business tax breaks we’re giving away, which we never look at to see if they’re actually accomplishing a purpose, he said. “There’s not one on the top of my mind right now, but this is an issue I looked at when I was secretary of administration and finance, and there are certainly areas that we haven’t even looked at.”

Gonzalez is challenging fellow Democrat Bob Massie in a September primary — the winner hopes to unseat incumbent Gov. Charlie Baker in the upcoming November election.

Baker declined to take a position on the so-called ‘millionaires tax’ ballot question.