Denver, CO (LifeNews.com) -- Colorado abortion advocates staged a rally on Tuesday against a potential ballot proposal to amend the state constitution to define unborn children as persons under state law. The measure could set off a statewide battle over abortion, embryonic stem cell research and other practices that kill people before birth.

The pro-life group Colorado for Equal Rights has to submit 76,000 signatures by May 13 to qualify the measure for the ballot.

However, abortion advocates aren't waiting for the ballot battle to become official before condemning the proposal.

Members of the newly-formed pro-abortion group Protect Families, Protect Choices rallied on the capitol steps and claimed the measure would violate women's right to basic health care.

Mary Fairbanks, a doctor at St. Anthony's Family Medicine Residency, told the Denver Post she opposes protecting unborn children and appeared to not understand basic fetal development facts.

"'The moment of 'fertilization' is not a medical definition and is almost impossible to determine outside a petri dish," Fairbanks said. "I believe this would endanger the lives of women in Colorado."

Sen. Betty Boyd, a Democrat from Lakewood and one of 20 pro-abortion legislators who have joined the new group, claimed the amendment is "dangerous and misleading."

She claimed government officials would have to investigate miscarriages and the use of birth control to enforce the measure.

Pro-life advocates in Colorado are split on the idea and the Colorado Catholic Conference released a message in February saying it has not endorsed the idea.

Catholic Conference executive director Jennifer Kraska told the Denver Post that Pueblo Bishop Arthur Tafoya, Colorado Springs Bishop Michael Sheridan, and Denver Archbishop Charles Chaput are in a tough position.

She indicated they support the principle behind the personhood amendment, which recognizes unborn children as persons under law from conception on, but she said they have concerns over the strategy.

Kraska indicated they are worried that the Supreme Court, which has a pro-abortion majority, would declare the amendment unconstitutional and it would further entrench the Roe v. Wade decision allowing virtually unlimited abortions.

"We commend the goal of this effort to end abortion. Individual Catholics may choose to work for its passage," Kraska said. "At the same time, we recognize that other people committed to the sanctity of life have raised serious questions about this specific amendment's timing and content."

Colorado for Equal Rights organizer Keith Mason told LifeNews.com at the time that "over 274 churches have signed on in support of personhood, over 856 petition gatherers across Colorado are working hard to make a difference."

The personhood amendment states "(t)he term 'Person' or 'Persons' shall include any human from the time of fertilization."