On Wednesday, citing concerns over that intrusiveness, McDonnell (R), an abortion opponent who had repeatedly said he would sign the bill, asked state lawmakers to amend the measure. House members approved the governor’s amendments, but the bill’s Republican sponsor in the Senate said she would try to pull the measure for the session.

“Mandating an invasive procedure in order to give informed consent is not a proper role for the state,” McDonnell said in a statement. “No person should be directed to undergo an invasive procedure by the state, without their consent, as a precondition to another medical procedure.” He did not comment further.

Confusion over the legislation and ultrasounds — and considerable national media attention — preceded the unraveling of the bill. The original measure stated, simply, that a woman needed an ultrasound before an abortion. Many lawmakers did not understand that at the young fetal age abortions usually occur, the invasive vaginal ultrasound would be needed to establish gestational age, as required by the bill.

McDonnell, who is often mentioned as a possible presidential running mate, tried to strike a balance between supporting his conservative base and supporting a bill that immediately drew opposition as a severe end-run against abortion rights.

“Bob McDonnell’s political future is not enhanced by vaginal ultrasound legislation,” said Stephen Farnsworth, a professor of political science at the University of Mary Washington. “But make no mistake about it: Governor McDonnell was painted into this corner by his own Republican legislature. This is an example of that old adage, ‘Be careful what you wish for.’ ’’

Republican lawmakers on Wednesday in essence said that an abdominal — or “jelly-on-the-belly” — ultrasound before an abortion would still be required but that vaginal ultrasounds would be voluntary.

The decision was a rare — and small — victory for Democrats and abortion-rights supporters after Republicans took control of both chambers of the General Assembly last month.

Democrats tried to kill the bill, saying the amendments only made the legislation worse, but Republicans cut off debate.

“Remember how we got into the mess to begin with,” said Del. Jennifer L. McClellan (D-Richmond). “Because when legislators wrote a bill, they didn’t understand the medicine.”

The measure now heads to the Senate, which had passed the unamended version once. Sen. Jill Holtzman Vogel (R-Winchester), who introduced the measure, said she did not realize that the ultrasound would not be external, and she said she still was not certain that would be required under the legislation. But she said she would move to strike her bill because of that uncertainty. That would require a Senate vote. Another version of the bill has been passed by the House and awaits Senate action.