Washington (CNN) – President Obama nominated Solicitor General Elena Kagan to the Supreme Court on Monday, picking her to fill the seat being vacated by retiring Justice John Paul Stevens.

If confirmed, Kagan would be the third woman on the nine-justice bench and the fourth in the history of the high court. Her confirmation also would mean that the Supreme Court would have no Protestant justices for the first time in its history. Kagan, who is Jewish, would join six Catholic and two Jewish justices.

CNN senior legal analyst Jeffrey Toobin has been a friend of Kagan's since college. He joined us on Monday's American Morning to discuss her nomination.

[cnn-photo-caption image=http://i2.cdn.turner.com/cnn/2009/images/10/05/am.intv.toobin.art.jpg caption="Jeffrey Toobin says Justice Sotomayor will probably vote very much the way Justice Souter did."]

The Supreme Court starts its new term this morning. Justice Sonia Sotomayor will make her debut on the bench and there are also some critical cases on the docket to talk about.

CNN Senior Legal Analyst Jeffrey Toobin, author of the best-selling book "The Nine, spoke with John Roberts on CNN's "American Morning" Monday.

John Roberts: So what impact do you think Justice Sonia Sotomayor is going to have on the court and how do you think she'll be different than Souter was?

Jeffrey Toobin: Initially, probably not that much. I think she'll probably vote very much the way Souter did. She seems to be a moderate liberal, he was a moderate liberal. But over time there certainly could be an influence. You know, the liberal block of the court has been pretty old in recent years. Justice Stevens is 89 years old, Justice Ginsburg is 76. The fact that there is this injection of new blood that she's only in her mid-50s. Justice Stevens likely to leave, likely to be replaced by President Obama with another liberal, that could generate some force on the liberal side even though they are basically outnumbered.

Roberts: Hard to think that someone in their mid-50s could be considered new blood.

Toobin: Your name sake, the Chief Justice Roberts, I loved the way they always talked about him, he's so young, so young. Good for us.

Roberts: People obviously will be looking for rookie mistakes to be made, but she's got 17 years on the bench, she proved herself when they had that rare September hearing on the Hilary Clinton movie, that she's not just going to just sit back and let the other ones take the lead she jumped in there and asked a lot of questions.

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[cnn-photo-caption image= http://i2.cdn.turner.com/cnn/2009/images/07/15/art.toobin.cnn.jpg caption="CNN Legal Analyst Jeffrey Toobin calls Judge Sonia Sotomayor 'a cautious, careful liberal.'"]

By Jeffrey Toobin
CNN Senior Analyst

(CNN) - One of the enduring myths about Supreme Court justices is that they often turn out to "surprise" the presidents who appoint them. Sure-thing conservatives, it is said, turn out to be liberals, ­and vice versa. In fact, the evidence is almost entirely the opposite: that with justices, as in life, what you see is what you get.

The question, then, is this: What do you see when you look at Sonia Sotomayor, who begins her confirmation hearings as a strong favorite for confirmation?

She is, above all, a veteran judge ­who has 18 years on the federal bench: six as a trial judge (appointed by President George H.W. Bush) and the rest on the court of appeals (appointed by President Clinton). The question of competence is closed. Sotomayor can do the job. It's no surprise that she received a unanimous rating of well-qualified from the American Bar Association screening committee.

But what would she stand for as a Supreme Court justice? She is, it seems, a liberal,­ but a liberal in the cautious and careful mode of her likely future colleagues Ruth Bader Ginsburg and Stephen Breyer.

Her leanings are clearest in the case of affirmative action. As a political and constitutional matter, she believes government can take steps to assure a diverse work force or student body.

This view was on display in the most famous (or infamous) decision of her career.

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