DEMOCRATIC GUBERNATORIAL candidate Sonia Chang-Diaz, frustrated in her attempts to negotiate a pre-convention, one-on-one debate with her rival Maura Healey, used a Wednesday night forum on climate change to accentuate what she perceives as key differences between their campaigns.

Sitting next to the attorney general and often addressing her directly, Chang-Diaz, the state senator from Jamaica Plain, criticized Healey, the state’s attorney general, for accepting campaign contributions from fossil fuel interests, for shortchanging public transit in her climate change plan, and for repeatedly failing to stand up for people of color at key moments in Beacon Hill policy debates.

The forum also highlighted a key philosophical difference between the two candidates. Chang-Diaz said she believes some sort of carbon tax is needed to steer consumers away from fossil fuels and to fund climate change initiatives. Healey, by contrast, said she wasn’t ready to support a carbon tax and instead preferred the cap-and-invest approaches of the Regional Greenhouse Gas Initiative and the Transportation Climate Initiative.

RGGI is a consortium of states, including Massachusetts, that place an assessment on electricity produced using fossil fuels and use the proceeds to fund energy-saving programs. The Transportation Climate Initiative was a regional initiative pushed by Gov. Charlie Baker to place an assessment on gasoline and use the proceeds to combat climate change. Baker withdrew TCI last year after no other states would join.

Forums tend to be low-key, wonky affairs where candidates explain their views and rarely interact with each other. That’s the way the climate change forum, sponsored by the Environmental League of Massachusetts and WBUR, started out. But about halfway through the event Chang-Diaz, who is trailing far behind Healey in the polls, began to address the attorney general directly, and moderator Tiziana Dearing did nothing to stop her. In fact, Dearing urged Healey to respond to Chang-Diaz’s attacks, turning the forum into something closer to a debate.

Chang-Diaz said she pledged not to accept campaign contributions from oil, gas, and coal executives, their lobbyists, or their PACs. She urged Healey to make a similar pledge and give back $50,000 in campaign contributions Chang-Diaz said attorney general received from fossil fuel interests since her last election.

Healey dismissed the allegation. “I don’t think the fossil fuel industry likes me very much. Exxon Mobil took me to court no less than three times in three different states to try to shut down my investigation [of the company’s concealment of climate change information.] I am not on their holiday card list.”

Chang-Diaz returned to the issue a short time later. “Maura, I respect the work that you have done against Exxon Mobil. I believe you are not on their Christmas card list,” she said. “But there are a lot of oil and gas utilities here in Massachusetts whose donation lists you are on. There is $50,000 worth of fossil fuel donations on your campaign donation rolls since your last election and I think it’s important that we make this pledge so voters know where we stand.”

Healey said she didn’t know what Chang-Diaz was referring to. She pointed out that both she and Chang-Diaz are committed to fighting climate change, unlike the two Republican candidates for governor — Geoff Diehl and Chris Doughty — who declined to attend the forum.

Chang-Diaz said she supports large-scale investment in electrification of the state’s transportation sector and investments in public transit, including construction of East-West rail and elimination of all fares on the MBTA and all regional transit authorities.

“This is an honest difference between the attorney general’s plan and mine,” Chang-Diaz said. “We both talk about our transportation sector needing to make investments in electric vehicles and electric vehicle charging. Maura makes a big bet on electric vehicles but there is not a similar big bet in her plan on transit.”

Sen. Sonia Chang-Diaz and Attorney General Maura Healey at a forum on climate change sponsored by WBUR and the Environmental League of Massachusetts.

Chang-Diaz said she has heard Healey talk about centering her policies around racial justice, which she applauded. But she charged that Healey has not always backed up her words with actions.

“The truth is there are many moments that people of color needed you to be there to center racial equity when you did not,” Chang-Diaz said.

For example, the senator said, Healey supports widespread adoption of electric vehicles but her climate plan offers little relief to those who are transit dependent. She said Healey also failed to step up in support of the Student Opportunity Act and police reform legislation,

“There are legislators who, frankly, hid behind you to excuse their vote against the police reform bill in this state and you did not say anything when they hid behind your letter that you wrote to the conference committee on that police reform bill,” Chang-Diaz said.

“I respect the hell out of your work on the national level,” Chang-Diaz said. “But standing up for racial equity at every turn is more than doing it when it’s against Donald Trump or against Exxon Mobil. The next governor is going to need to have the courage to do that when it’s against members of our own party in the Legislature or when it’s in opposition to wealthy donors.”

Healey defended her record. “I obviously disagree with what are mischaracterizations of my record on the education front and a number of fronts,” she said. Healey said she supports East-West rail and the electrification of public transit, but did not say whether she favors eliminating fares at the MBTA and at regional transit authorities.