The governor of Virginia has reined in his previously unconditional support for a bill which would force women seeking abortions in the state to undergo an internal ultrasound.

Governor Robert McDonnell, an anti-abortion Republican who is seen by some in his party as a possible candidate for vice-president, had previously said he would sign the bill if it were passed by Virginia's general assembly.

But after mounting pressure from campaigners, Democratic delegates and ordinary Virginians who strongly oppose the measure, McDonnell will now say only that he will review the bill if it appears on his desk.

This week, Virginia has come under increasing national scrutiny for moves to pass the legislation, the first of two of the most controversial anti-abortion bills in recent history proposed by state Republicans. It would force women undergoing first trimester abortions – when the majority of abortions are performed – to submit to a vaginally invasive procedure, offer them images of the foetus and have the resulting image lie on their medical file for seven years.

Democratic delegates say if it passes it risks criminalising doctors by forcing them to carry out an invasive procedure that could constitute a sex crime under state laws.

The reason for the governor's change of mind is unclear, although two officials told the Washington Post that a compromise was likely after some of the bill's supporters were apparently unaware of how invasive the procedure could be.

Charniele Herring, a Democratic delegate who tried to kill the bill on Tuesday, said it was "disappointing" if the bill's supporters were unaware of what they were voting on.

She said: "In the full questions of justice committee where it went through first, everybody got a very graphic picture of what a transvaginal ultrasound is. A picture showed exactly what it was. I'm not in the Republican caucus but it's possible that those who saw it did not share it with their Republican colleagues. It is very disappointing because we are legislators and we should know what we are voting for."

She believes that McDonnell's change of heart is politically motivated.

"He is trying to paint himself as a moderate and this is extreme. There is a clamour among Virginians for him to say that he will not sign it in its current form."

The bill, which is due to be heard by the general assembly again on Wednesday, has been passed by for three consecutive days amid media reports which have drawn attention to the issue nationally.