In a surprising move that may level the Democratic playing field in the Louisiana open-seat Senate race, Rep. William Jefferson (D-La.) endorsed the candidacy of state Treasurer John Kennedy (D) late Tuesday.

“I believe John Kennedy presents our best hope to defeat [Rep.] David Vitter [R],” Jefferson said in a statement. “John Kennedy has already proven that he can build coalitions that win statewide elections for Democrats — pulling workers, African-Americans, women and progressive Democrats together to win two statewide elections.”

Kennedy, who has twice been elected to his current post, praised Jefferson in a statement of his own.

“Congressman Jefferson has first-hand knowledge of what makes an effective and committed United States Senator for Louisiana,” said Kennedy. “I am very humbled to know he sees these qualities in me.”

The Jefferson endorsement is seen as a setback for Rep. Chris John (D), who to this point has secured nearly every institutional endorsement available. Aside from John, Kennedy and Vitter, state Rep. Arthur Morrell (D) is also competing in the Nov. 2 open primary.

To this point, Vitter and John have long been considered shoo-ins for an expected Dec. 4 runoff, which will be triggered if none of the candidates receive 50 percent in the primary.

John has been endorsed by retiring Sen. John Breaux (D-La.) as well as the Democratic Senatorial Campaign Committee, which usually does not get involved in contested primaries.

DSCC Chairman Jon Corzine (N.J.) largely dismissed news of the Jefferson endorsement Tuesday, saying it “wouldn’t be helpful, but I certainly don’t think it’s critical.”

John allies also noted that Jefferson’s endorsement of ex-Rep. Buddy Leach (D) in the 2003 gubernatorial race had little effect on the outcome.

In that race, Jefferson endorsed Leach less than two weeks before the state’s October open primary in what was at the time seen as a coup for the former Congressman.

Leach placed fourth overall, and third among Democrats with 14 percent.

Despite the pushback from the DSCC and John, the Jefferson endorsement was welcome news to Kennedy’s campaign, which has struggled to break the Congressman’s lock on institutional Washington.

It also provides Kennedy with a boost in the black community, the most influential voting bloc for Democrats seeking statewide office in Louisiana.

While blacks make up roughly 33 percent of the state’s overall population, they are a much larger segment of the Democratic electorate.

Recent Louisiana political history reveals an uneasy relationship between establishment Democratic choices and the black community.

In 2002, former state Rep. Cleo Fields (D), a major figure in the black community, contemplated a challenge to Sen. Mary Landrieu (D) before ultimately bowing out and offering a tepid endorsement.

Fields was angry that Landrieu had not eagerly backed him when he narrowly beat her out for a place in the 1995 gubernatorial runoff against eventual Gov. Mike Foster (R).

Landrieu was unable to avoid a December runoff in 2002 but survived that contest.

Morrell, who is black, was initially expected to be the choice of African-American leaders but has shown little ability to raise the funds necessary to run a serious campaign.

John remains the best funded of the Democratic candidates with $2.4 million in the bank as of July 17. He has used that financial edge to fund a large statewide television buy that, his campaign maintains, has moved him past Kennedy and into second place in internal polling.