U.S. Sen. Susan Collins is coming under unusual fire from women’s health groups for supporting a Republican-backed effort to end federal funding for Planned Parenthood.

But Collins’ office said the Maine senator voted Monday for the defunding bill on procedural grounds after being assured by party leaders that her support would pave the way for an alternative measure she co-sponsored to maintain funding for most services while ordering an investigation by the Justice Department.

“Unfortunately, Senator Susan Collins surprised Maine women by backing away from previous statements and voting in support of defunding the country’s largest, most trusted women’s health care provider,” Planned Parenthood of Northern New England said in a written statement Tuesday.

The Mabel Wadsworth Women’s Health Center in Bangor, a nonprofit that provides a range of reproductive health services, also expressed disappointment in Collins’ action.

“Her vote seriously calls into question her appreciation for the economic challenges women face in accessing reproductive health care,” the center’s executive director, Andrea Irwin, said in a written statement. “For many women, Planned Parenthood is the only source of affordable care and serves as a vital safety net.”

Maine Democratic Party Chairman Phil Bartlett said in a written statement that Collins’ vote contradicts expressions of support she has made for the organization and its work on women’s health care issues.

“Susan Collins’ vote proves her allegiance to the Republican Party is more important than standing up for her constituents,” Bartlett said.

Planned Parenthood has been under intense criticism since anti-abortion activists released secretly recorded video of organization leaders discussing the use of fetal tissue from abortions for research purposes. That has resulted in allegations that Planned Parenthood profits from abortions and prompted a drive by Senate conservatives to strip the organization of its federal funding.

Monday’s 53-46 vote in the Senate came on a procedural move to block Democratic delays of the Republican defunding bill. The move failed because it fell short of the 60 votes needed.

Collins, who spoke against defunding the organization during her floor speech, said that her vote to advance the bill was designed to open the door procedurally for a separate bill she endorsed that would order a Department of Justice investigation into whether Planned Parenthood is profiting from fetal tissue donations.

Alleigh Marre, Collins’ spokeswoman, said Tuesday that the senator had received assurances from Senate Majority Leader Mitch McConnell, R-Kentucky, that supporting the defunding bill would allow Collins’ investigation bill to move forward. Collins, in other media reports, said that the defunding bill “goes too far” and would affect all Planned Parenthood clinics, not just those under suspicion of selling fetal tissue.

If the defunding bill advanced in its current form, she told the National Journal, she would vote against it.

POINTING OUT CONFLICTING POSITIONS

Collins was not endorsed by Planned Parenthood when she ran for re-election in November, but she has generally enjoyed at least measured support from women’s groups because of her moderate profile on women’s issues. The organization praised her in 2011 for opposing a move in Congress to defund Planned Parenthood as part of a continuing budget resolution.

The criticism leveled Tuesday was a departure for the groups, which suggested Collins is using a procedural explanation to maintain her reputation as a supporter of women’s health services, while she casts votes to defund an organization that received more than a third of its $1.3 billion in funding last year from the federal government.

Collins co-sponsored the investigation bill with Sen. Mark Kirk, R-Illinois, and Sen. Lisa Murkowski, R-Alaska. Murkowski also voted to advance the defunding bill, while Kirk voted against it.

The bill proposed by the trio of Republican senators was supported Tuesday by Republican Majority for Choice, a national pro-choice group that described the bill as a “thoughtful alternative” to defunding Planned Parenthood.

“The recent controversy brought on by anti-choice attacks on Planned Parenthood has raised enough questions to prompt a congressional investigation,” the group said. “As a pro-choice organization that has long supported Planned Parenthood, we believe an external investigation by the Department of Justice, as proposed by senators Collins, Kirk and Murkowski, would take a fair and balanced approach.”

During her floor speech Monday, Collins said she was “sickened when I viewed the recently released videos featuring Planned Parenthood physicians in both their edited and unedited versions. The callousness that the Planned Parenthood employees displayed in discussing the sale of fetal tissue is appalling.”

She then added, “We do, however, need to keep in mind the fact that Planned Parenthood provides important family planning, cancer screening, and basic preventive health care services to millions of women across the country. For many women, Planned Parenthood clinics provide the only health care services they receive. It would be premature to totally defund Planned Parenthood immediately until we know more facts.”

DEFUNDING ISSUE FAR FROM DEAD

According to the Congressional Budget Office, the defunding bill would likely have a mixed impact on federal spending. In a letter Monday to the chairman of the Senate Budget Committee, the CBO concluded that Medicaid spending for services that avert pregnancy and deliveries would be expected to decrease if the bill passed. But the letter also pointed out that reduced use of such services would presumably lead to additional births, which would increase the demand for Medicaid services, both by adults and the children they bear.

Planned Parenthood has said that its northern New England offices do not provide fetal tissue from abortions for medical research.

The organization praised Maine independent Sen. Angus King, who voted against the defunding measure and noted that 97 percent of Planned Parenthood’s services to women have nothing to do with abortion. “The net effect of this bill is simply to deny basic health care, including contraception, to millions of women, particularly low-income women,” King said.

Although the bill failed Monday, congressional Republicans have made the issue a centerpiece of their legislative agenda and have threatened to shut down the government if federal funding for the group isn’t curtailed. That means the Planned Parenthood funding issue is likely to surface again in the fall.