Republican presidential front-runner Mitt Romney, whose health-care record as governor of Massachusetts has left him struggling to win the support of conservative voters, now faces another point of vulnerability: his environmental record.

Just days after his 2002 election, Mr. Romney hired Douglas Foy, one of the state’s most prominent environmental activists, and put him in charge of supervising four state agencies.

Associated PressGov. Mitt Romney and Douglas Foy at a March 2006 event.

Mr. Foy had initiated a lawsuit that led to the cleanup of Boston Harbor and had worked to protect fishing grounds and seashores. Once in the Romney administration, he served as the governor’s negotiator on a regional climate-change initiative and helped draft regulations to put emissions caps in place for coal-fired power plants.

With Mr. Foy by his side, Mr. Romney joined activists outside an aging, coal-fired plant in 2003 to show his commitment to the emissions caps. “I will not create jobs or hold jobs that kill people, and that plant, that plant kills people,” he said.

Mr. Romney, while implementing the emissions caps, ultimately backed away from the regional climate-change agreement in 2005, a decision announced on the same day he said he would not seek re-election as governor, stoking speculation that he would run for president.

Now, Mr. Romney’s record on the environment is becoming fodder for rivals in the presidential race who are trying to stoke doubts about his commitment to conservative principles. On Friday, Texas Gov. Rick Perry launched a broadside against his rival’s environmental record, saying it showed he would govern like President Barack Obama.

“Massachusetts was one of the first states to implement its own cap and trade program, which included limits on carbon emissions from power plants,” Mr. Perry said in a speech outside Atlanta.

The Romney campaign accused Mr. Perry of using “distortions and fabrications” to tag Mr. Romney with policies either initiated by his predecessor governors or implemented by his successor, Democrat Deval Patrick.

As a candidate, Mr. Romney has laid out an economic plan that would amend the Clean Air Act to exclude carbon dioxide from its regulatory purview and open U.S. energy reserves to more development.

In his five-year quest for the presidency, Mr. Romney has been bedeviled not only by his Massachusetts health-care plan, which mandated that most individuals purchase insurance, but also his shifts on abortion and gun control. In 2004, he signed an assault weapons ban for his state, although he now opposes most gun control. And he promised as governor he would keep his state’s pro-abortion rights position, though now he opposes legal abortions.

Mr. Romney says that his health-care plan was good policy for Massachusetts, but that as president he’d work to overturn Mr. Obama’s health-care law, which also includes an individual mandate.

“He may very well emerge as the best choice, but he’s certainly not going to be the first choice of a number of Republicans,” John Ullyot, a GOP strategist and long-time Senate staffer, said of Mr. Romney. “And that stems from the inherent difficulty of being a Northeastern, successful Republican governor turning around and trying to appeal to a far more conservative electorate.”

A Quinnipiac University poll of Republican voters released Wednesday put Mr. Romney on top in the GOP race, but with only 22% of the primary vote. Former pizza company executive Herman Cain surged to second with 17%, followed by Mr. Perry with 14%.

Mr. Romney’s approach to the environment as governor showed someone who was open to regulatory as well as market-oriented answers to environmental problems—while also willing to work with committed environmentalists and liberal Democrats.

His record in Massachusetts is more complicated than the summary offered by Mr. Perry. Mr. Romney actually refused to join the regional cap-and-trade program his administration helped create. As governor, he angered business leaders and environmentalists alike. But that complexity may be part of his problem among voters seeking consistency or clarity.

From the start of his administration, Mr. Romney set out to reconcile a pro-business political bent with his state’s liberal environmentalism, said Eric Kriss, a close confidante of Mr. Romney’s from their days co-founding the private equity firm Bain Capital. During the Romney campaign for governor, Mr. Kriss consulted frequently with Mr. Foy.

“Doug was known as a pre-eminent conservationist,” he said. “He was broad-minded, articulate, and he believed in the vision we all had, to combine environmental concerns with the need for housing and transportation infrastructure.”

Mr. Foy was put in charge of commonwealth development, overseeing transportation, housing, environment and energy agencies, with combined annual capital budgets of $5 billion and more than 11,000 employees.

Mr. Foy, a political independent who votes Democratic, said he joined the Romney administration not to combat climate change but because the governor was promoting “smart growth” measures and combining warring parts of the state government into a single entity that Mr. Foy would supervise.

He said that Mr. Romney’s environmental record fits “the Republican tradition in Massachusetts” of “fiscal conservatism and good governance, doing more with less.”

Romney campaign aides said his efforts involving coal-fired plants were merely the tail end of a process launched by his predecessor, Republican Gov. Jane Swift, who in 2001 ordered the plants to reduce their emissions of pollutants and carbon dioxide.

Mr. Foy said the governor was not personally involved in drafting the emissions caps placed on the plants, but “he was certainly well aware” of the regulations.

On Dec. 7, 2005, the Romney administration unveiled the final orders. “These carbon emission limits will provide real and immediate progress in the battle to improve our environment,” then-Gov. Romney said in a press release touting Massachusetts as “the first and only state to set CO2 emissions limits on power plants.”

Mr. Foy said that as he was negotiating a cap-and-trade regime with other states, Mr. Romney made it clear he believed in human-caused global warming and wanted a policy response. At the time, many conservatives were open to a cap-and-trade system, seeing it as a market-driven solution to limiting emissions.

Still, Mr. Foy said, the governor peppered him with questions about the economic impacts of a carbon-trading system. At his urging, negotiators built business-oriented provisions into the agreement, such as triggers to cut off trading if the price of energy rose to certain levels.

But after instructing Mr. Foy to press forward to negotiate an agreement, Mr. Romney ultimately backed out, saying the deal lacked a “safety valve” to cap plant payments if they exceeded emission limits.

Write to Jonathan Weisman at jonathan.weisman@wsj.com