No one in politics would make the mistake of saying the vice president opposes abortion rights, but Joe Biden, the women's rights advocate and a staunch Catholic, has not been unwavering throughout his career in support of the right to choose.

Biden steps up tonight as the face of the Obama administration and the Democratic Party in the debate against Republican vice presidential nominee, Paul Ryan.

After the first in the series of presidential debates last Wednesday, some on the left complained that the candidates did not discuss their views on abortion.

Both camps have released ads attacking each other's positions on contraception, Planned Parenthood funding and abortion.

Ryan's views are unmistakably far to the right on the issue of abortion. While Mitt Romney has evolved during his political career from a supporter of abortion rights to an opponent of them, Ryan once supported a bill outlawing abortion that - unlike most laws - did not explicitly make an exception for cases of rape and incest.

But both Ryan and Biden believe that life begins at the moment of conception.

"Look, I know when it begins for me," Biden told Tom Brokaw in 2008, saying he'll take his cues from the Roman Catholic church on this question. "I'm prepared as a matter of faith to accept that life begins at the moment of conception. But that is my judgment. For me to impose that judgment on everyone else who is equally and maybe even more devout than I am seems to me is inappropriate in a pluralistic society."

In a 2007 interview with Tim Russert, Biden acknowledged his views on abortion had changed since he came to the Senate at age 29.

Biden reflects his nuanced views in his voting history in the Senate.

While he voted to establish fines and penalties for barring access to abortion clinics, he repeatedly voted to ban so-called "partial-birth abortions." There were other times when abortion legislation was on the table that he simply didn't vote.

Both Biden and Obama voted against a bill making it illegal to take a pregnant minor to another state to get an abortion in order to get around parental consent laws, but Biden opposes letting taxpayer dollars go to abortions.

Biden acknowledged his political middling on this point in his book "Promises to Keep: On Life and Politics."

"I've stuck to my middle-of-the-road position on abortion for more than thirty years," Biden wrote in 2007. "I still vote against partial birth abortion and federal funding, and I'd like to find ways to make it easier for scared young mothers to choose not to have an abortion, but I will also vote against a constitutional amendment that strips a woman of her right to make her own choice. That position has earned me the distrust of some women's groups, and the outright enmity of the Right to Life groups."

Just this week, the Obama campaign called into question Republican nominee Mitt Romney's views on abortion when he told the Des Moines Register that he knew of no abortion legislation that he would include in his agenda.

A day later, Romney pivoted away from that stance, saying he is a "pro-life candidate."

Read More About Mitt Romney's Evolution on Abortion

"I'll be a pro-life president," Romney said. "The actions I'll take immediately are to remove funding for Planned Parenthood. It will not be part of my budget."