The search is on for Sister Souljah. In 1992, Governor Clinton, then the leader of Arkansas, denounced Sister Souljah, a rapper and activist with a penchant for the extreme. Ever since then it has been commonplace for pundits to ponder the “Sister Souljah Moment” of major candidates, the opportunity when a politician can move himself to the center by chastising somebody further out on the periphery of their own movement.

Talk of Sister Souljah is back in the news because of MoveOn.org’s ad impugning the honor of General Petraeus. On September 6, Kenneth Blackwell raised the possibility of Senator Clinton having a Sister Souljah Moment and denouncing one of the liberal groups which are supportive of her candidacy. Fox News’s Neil Cavuto asked Mayor Koch about Mrs. Clinton and the MoveOn.org ad, “Why didn’t Hillary Clinton have … a Sister Souljah moment, and just say, wrong, you have pushed it too far?” Mayor Koch responded, “She should have.” Commentator on the blog Jam Side Down, Martin Manley, reversed the equation and suggested that the MoveOn.org ad created an opportunity for Mr. Obama to have such a moment, but don’t lose sleep waiting for Mr. Obama to turn against the progressives in his party. He can’t. They are providing his campaign with the energy and money that are keeping him a relevant candidate. In an odd political symbiosis, Mr. Obama’s presence in the primaries helps, not hurts, Mrs. Clinton.

While Mr. Obama is far from the kind of extreme and polarizing figure that Sister Souljah was in 1992 and avoids the vitriol of MoveOn.org, his political positioning to the left of Mrs. Clinton serves a similar role — to place Mrs. Clinton in the center. Mrs. Clinton still needs to defeat the skilled and well-financed Mr. Obama, but when and if she does, her victory over him will in and of itself be important as a marker of how far to the left the party won’t go.

Consider the times Mrs. Clinton has scored points against Mr. Obama. In the April debate in South Carolina, the Illinois senator was asked what he would do as president if terrorists attacked two American cities simultaneously. Mr. Obama answered, “The first thing we’d have to do is make sure that we’ve got an effective emergency response, something that this administration failed to do when we had a hurricane in New Orleans.” Mrs. Clinton struck back immediately and said, “I think a president must move as swiftly as is prudent to retaliate.”

When, at the YouTube debate in July, Mr. Obama rushed to commit to meet with the leaders of Iran, Syria, Cuba, North Korea, and Venezuela during his first year in office. Senator Clinton again showed judgment and skill. She responded, “I don’t want to be used for propaganda purposes. I don’t want to make a situation even worse.” She punctuated the moment in the days that followed, terming Mr. Obama’s response as “irresponsible and frankly, naïve.”

As long as Mr. Obama stands as a credible challenger for the Democratic nomination to Mrs. Clinton, he does something else for her. Mr. Obama shields Mrs. Clinton from the scrutiny of the conservatives and the national press. When a candidate is designated as a complete front-runner, that candidate immediately develops a target on their back.

The history of Democratic Party is littered with the political corpses of onetime front-runners — Senators Edmund Muskie and Edward Kennedy, Governor Howard Dean, to name but a few. If there is at least one other candidate to take bullets, the damage that can be sustained is oddly diminished. While Mrs. Clinton must withstand her share of criticisms and attacks from Mr. Obama and the other candidates, Mr. Obama’s mere presence in the primaries serves to diffuse the scrutiny on her and her campaign.

Without Mr. Obama to take his share of criticism this month, the story of the arrest of Norman Hsu, who helped raise an astonishingly large amount of money for Mrs. Clinton, would have, if anything, received even more play. There will be difficult moments ahead. The well-financed Mr. Obama will give Mrs. Clinton a run for her money. Yet by the time the general election arrives, Mrs. Clinton will be happy to have had Mr. Obama on the far left flank.

Mr. Gitell (gitell.com) is a contributing editor of The New York Sun.