The ad, which was airing Tuesday in Iowa, features three women, one of whom talks about what she felt when she saw her fetus on ultrasound. "I mean, there was my tiny child's heart beating, arms waving," she says.

Forbes ends the ad, his first on abortion, by saying: "I want to protect the rights of all Americans, including the unborn, but I need your support to make this happen."

When he ran in 1996, Forbes' campaign views on abortion were considerably more moderate. This year, as he has sought votes from religious conservatives and tried to broaden his message beyond the flat tax, he has adopted a hard-line anti-abortion stand.

In a Republican debate Monday, Forbes challenged Bush to promise that if he is the GOP nominee he will pick an anti-abortion running mate, appoint only anti-abortion judges and preserve the anti-abortion plank in the Republican platform.

Bush declined all on three fronts.

"I'm going to pick a vice president who can be the president. I'll pick judges who strictly interpret the Constitution and not use the bench ... to legislate. And I will ... work to keep the Republican Party pro-life," Bush said.

Forbes called that response "a typical hedge" and said, "We need something specific."

Bush, looking ahead to the general election, doesn't want to alienate the moderate voters he will need if he's the GOP nominee. Forbes, trying to break through, is courting conservatives.

The new ad does not mention Bush, or any other candidate, by name.

"It demonstrates Forbes' commitment to the pro-life cause," said Keith Appell, a Forbes spokesman. "He will use the bully pulpit to advance this issue forward."