A protester contends that the pipeline could add to global warming and damage Arctic ecosystems during an Nov. 2011 protest. (Evan Vucci/AP)

Opponents of the proposed Keystone XL oil pipeline called again Friday on the Obama administration assess the project's potential affects on public health before reaching a final decision. The request comes amid growing public and political support for the proposed pipeline between Canada and the United States.

Sens. Barbara Boxer (D-Calif.) and Sheldon Whitehouse (D-R.I.), two of the Senate's most liberal members and most ardent environmentalists, originally requested a public health study in late February but announced Friday that groups including the American Public Health Association and the National Association of County and City Health Officials were joining their push for the study. They want the study completed before Secretary of State John F. Kerry makes a final decision on whether the project should proceed.

"We feel that children and families in the United States have a right to know how greatly expanding the importation of tar sands into our country is going to affect their lives," Boxer told reporters as she announced the partnership with health professionals. She bemoaned the fact that the pipeline would transport "830,000 barrels of the dirtiest kind of oil every day" into the country, with most of it set for export to other countries.

Whitehouse said it would be wrong for the project "to be steamrolled through Washington, and we’re counting on Secretary Kerry and the president to take that careful look and ensure that we’re making the right decision here."

Boxer and Whitehouse's request appears to be a last-ditch effort to stave off growing political support for the project. A recent Washington Post-ABC News poll showed that Americans overwhelmingly favor building the pipeline by a nearly 3-to-1 margin. And this week 11 Senate Democrats -- six of whom face challenging reelections this year -- called on the administration to quickly approve the project.

Approving the project might help boost House and Senate Democratic candidates who favor building the pipeline, but it would infuriate environmentalists, a loyal Democratic constituency that this year is prepared to spend hundreds of millions of dollars supporting Democratic candidates with strong environmental records.

Asked Friday about her Democratic colleagues in tough reelections who favor building the pipeline, Boxer said: "They’ll do what they have to do to represent their states well and follow their conscious. What we’re doing is just saying that, when it comes to public health and the survival of the planet, you need to pay attention to that, whether it’s an election year or an off-election year."

A final decision from Kerry is expected in the coming months.