AUGUSTA — The early support that Republican U.S. Sen. Susan Collins has received from national groups that typically back liberal candidates signals the uphill battle Democrat Shenna Bellows faces in unseating the popular incumbent, political observers say.

Collins’ endorsement from a powerful environmental group, the League of Conservation Voters Action Fund, on Monday came on the heels of an announcement from the Human Rights Campaign, the nation’s largest LGBT advocacy organization, last week.

Collins’ campaign says the endorsements prove that the traditional liberal coalition isn’t lining up behind Bellows. But the former head of the American Civil Liberties Union of Maine contends that national organizations have a stake in keeping an incumbent in office and that the local support she’s getting is what matters.

“Certainly, endorsements by D.C.-based groups by Susan over myself do indicate that the odds are always in an incumbent’s favor,” she said. “But we are really excited about the grassroots movement that we are building.”

The endorsements from the groups, which have supported Collins in past elections, likely mean they acknowledge the long odds Bellows faces in defeating the three-term moderate Republican, observers say. A recent poll showed Collins leading by 55 percent.

“It makes sense to try to maintain a relationship with (Collins), assuming that they are confident that she will vote in the way that supports their interests,” said Ron Schmidt, a political science professor at the University of Southern Maine.

The endorsements in the race have come early and been a back-and-forth battle for support.

When Bellows announced her endorsement from one of the state’s largest labor groups, Maine AFL-CIO, Collins ran TV ads the same day highlighting her approval from four unions at Bath Iron Works.

Days after Bellows marched in the Portland Pride Parade, Collins touted her endorsement from the Human Rights Campaign. Bellows, who led Maine’s campaign in support of same-sex marriage in 2012, criticized Collins for announcing that she supported gay marriage only after earning HRC’s endorsement last week.

Collins’ campaign says the recent endorsements and others events, like the Maine Education Association’s decision not to support anyone in the race, show that the traditional Democratic campaign building blocks aren’t there for Bellows.

Steve Abbott, Collins’ campaign manager, said they also highlight the Republican’s ability to work with people across the political spectrum.

“As the environment in Washington becomes increasingly toxic and increasingly polarized, people are really responding to that message,” he said. “We are seeing that from a lot of these national groups who traditionally might support candidates on the other side of the aisle.”

Bellows’ campaign calls the endorsements part of the “incumbent protection machine” and says they don’t matter to Maine voters. Furthermore, these groups are “grading Republicans on a different scale,” Bellows said.

The League of Conservation Voters endorsed Collins, who has a 67 percent lifetime rating with the group, but conducted a $150,000 mailer campaign attacking Democratic congressional candidate and state Sen. Troy Jackson, who earned a 64 percent rating.

Tiernan Sittenfeld, senior vice president of government affairs for the organization, said it’s unfair to compare the ratings of the two candidates, who voted on different measures, and that Collins is by far the best Republican member in the Senate on environmental issues.