Nebraska lawmakers began a second round of debate Thursday on a bill that would ban abortion once cardiac activity can be detected in an embryo, which generally occurs around the sixth week of pregnancy and before most women know they are pregnant.

Supporters advanced the so-called heartbeat bill from the first round of debate earlier this month with only a one-vote margin to break a filibuster. The bill must survive Thursday’s debate and a final round to pass. But the effort in the Republican-controlled state remains in question.

An amendment introduced Thursday by a Republican co-signer to the bill, Sen. Merv Riepe of Ralston, would extend the proposed ban to 12 weeks and add to the bill's list of exceptions any fetal anomalies deemed incompatible with life.

NEBRASKA LAWMAKERS ADVANCE 'HEARTBEAT BILL' THAT BANS ABORTION AFTER CARDIAC ACTIVITY IS DETECTED

The bill includes exceptions for cases of rape, incest and medical emergencies that threaten the life of the mother, making specific exceptions for ectopic pregnancies, IVF procedures, and allowing for the removal of a fetus that has died in the womb. It also does not ascribe criminal penalties to either women who receive or doctors who perform abortions. Instead, it would subject doctors who perform abortions in violation of the measure to professional discipline, which could include losing their medical licenses.

Opponents of the bill were reluctant to take a stand on the amendment, focusing instead on concerns that the bill's language is ambiguous and could make medical professionals subject to criminal penalties — in particular a 1977 state law that makes abortion performed outside of accepted medical procedures a felony.

"Doctors are not going to have an adequate opportunity to know what's going on with this law," Sen. John Cavanaugh said Thursday.

The bill's author, Thurston Sen. Joni Albrecht rejected that argument, saying the bill is the "friendliest" abortion ban to the medical community in the country. But she rejected a compromise bill introduced by Omaha Sen. Jen Day that would explicitly exempt women and medical professionals from criminal penalties associated with an abortion.

"This is simply not necessary," Albrecht said. She also rejected Riepe's amendment, objecting to giving pregnant women 12 weeks to get an abortion because her 6-week proposal "was a big compromise" from the total abortion ban — which had no exceptions for rape or incest — she introduced and failed to get passed last year.

"This bill is about one thing," she said. "It's protecting babies with beating hearts from elective abortion."

The amendment and reports of support for it by some lawmakers who voted for the bill earlier this month could signal that a ban set very early in pregnancy may face pushback even from those who want further abortion restrictions.

NEBRASKA LAWMAKERS BEGIN DEBATE OVER ABORTION BAN

Nebraska has the only single-chamber, officially nonpartisan legislature in the United States. But each of its 49 lawmakers identifies as Republican or Democrat and tends to propose and vote for legislation along party lines. Republicans hold 32 seats, while Democrats hold 17 seats. Although bills can advance with a simple majority, it takes a supermajority — 33 votes — to end debate to overcome a filibuster. So a single lawmaker breaking from the party line could decide whether a bill advances or dies for the year.

The close divide played heavily in the defeat last year of Albrecht's so-called trigger bill that would have automatically banned nearly all abortions in the state as soon as the U.S. Supreme Court overturned Roe v. Wade, which had guaranteed the right to abortion nationwide for nearly five decades. That bill fell two votes short.

In the vote to advance the abortion bill earlier this year, Sen. Mike McDonnell, a Democrat, voted with Republicans. His reason, he said, is that he is a devout Roman Catholic who has always campaigned as an anti-abortion candidate.

Nebraska Gov. Jim Pillen, a Republican, has been a vocal proponent of the bill and has said he will sign it if it passes.