WASHINGTON — Under cover of Pope Francis’ arrival in the United States, Hillary Rodham Clinton on Tuesday came out against the Keystone XL Pipeline, a contentious environmental issue that has dogged the Obama administration.

Clinton announced her opposition to the deal just as TV and cable networks were focusing on Francis’ touchdown at Andrews Air Force Base in Maryland for the start of his historic US visit.

“I think it is imperative that we look at the Keystone Pipeline,” Clinton said.

“I believe it is a distraction from the important work we have to do to combat climate change, and, unfortunately from my perspective, one that interferes with our ability to move forward.

“Therefore, I oppose it. I don’t think it’s in the best interest of what we need to do to combat climate change,” she concluded.

Clinton made the announcement in Iowa, where she also unveiled a plan to cap prescription-drug costs at $250 a month and to deny tax breaks for drug companies’ TV advertising.

President Obama also has spoken critically about the oil project, even as his State Department continued its multi-year review of environmental issues connected to the pipeline, which would go from Canada through the US to refineries on the Gulf Coast.

Environmentalists fret that the project will contribute to global warming and threaten underground aquifers.

Clinton hadn’t said much about the pipeline as a candidate, but last week warned, “I’m putting the White House on notice. I’m going to tell you what I think soon, because I can’t wait.”

Rival Sen. Bernie Sanders opposes it.

The Teamsters union backs the pipeline, as do most Republicans, who have slammed the Obama administration for not boosting a project that would create many jobs.