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NASHUA, N.H. — Hillary Rodham Clinton has an aggressive set of proposals to combat climate change. And she has a pipeline problem.

The day after Mrs. Clinton put forth a detailed plan that would go further than any president in using the government to protect the environment and produce solar and wind energy, she found herself once again stumped on the question of whether or not she would support the Keystone XL Pipeline project, which would bring Canadian oil to Texas.

A voter at a Nashua town hall, Bruce Blodgett, a software engineer in Amherst, asked Mrs. Clinton to give a “yes or no” answer to whether she would support the pipeline, which liberals and environmentalists abhor. Mrs. Clinton demurred.

“This is President Obama’s decision and I’m not going to second-guess him,” she said. “If it’s undecided when I become president, I will answer your question.”

The response left a silence in the un-air-conditioned gymnasium of the Amherst Street Elementary School, where 450 people wiped sweat and sipped bottles of lukewarm water to get a chance to ask Mrs. Clinton a question. At her last town hall event in New Hampshire, climate change protesters heckled Mrs. Clinton about her stance on the environment.

By the time the national news media had its chance to ask Mrs. Clinton about Keystone, the Democratic candidate seemed exasperated.

“I will not do it,” she said when a CNN reporter pressed her on the pipeline. “I am sorry if people want me to.”

Mrs. Clinton reminded the audience, and the news media, of the proposals she presented in Iowa a day earlier. The unspoken lament: How about some credit for a comprehensive climate-change plan — including going further than the Obama administration in producing 33 percent of the nation’s electricity from renewable sources by 2027?

“I think to signal that there is only one overriding threat really doesn’t take into account the seriousness of a whole range of issues?” Mrs. Clinton said. She noted China, for instance, and its coal-fired plants, suggesting people not forget about those.

But Keystone nevertheless dominated the day. Two of Mrs. Clinton’s Democratic opponents, former Gov. Martin O’Malley of Maryland and Senator Bernie Sanders of Vermont, have both said they oppose the Keystone project.

“We’ve had dozens of pipelines already crossing our border from Canada, so we have to look at all of this,” Mrs. Clinton said. “That’s why I’m coming out with a comprehensive clean energy plan.”

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Keystone isn’t the only sticky issue Mrs. Clinton has punted on, attracting ire from the liberal wing of the Democratic Party, along with some tricky questions at town-hall events in this early nominating state.

On Tuesday, Mrs. Clinton reiterated that she would not comment on the Trans-Pacific Partnership agreement, the trade deal that Mr. Obama has championed, which ignited opposition from many Democrats who worry the agreement would hurt American workers.

“I’m in a different position than any other presidential candidate — I served in this administration for four years,” Mrs. Clinton said, adding that the negotiations over the trade deal and the Canadian pipeline were continuing, and that she had been involved in those discussions as secretary of state.

“If you look at both the trade deal and the Keystone decision,” Mrs. Clinton said, “I will certainly express my opinion when there is something to express an opinion on.”

In the meantime, she repeated, “I do not think it’s appropriate for me to comment on something I had official responsibilities for until it is completed and that I might have official responsibilities for again.”