U.S. Rep. Jim Clyburn will deliver South Carolina’s most coveted presidential endorsement Thursday when he announces his backing for his longtime congressional colleague Dick Gephardt in the state’s critical Feb. 3 Democratic primary.

The endorsement will deliver a much-needed boost for the Democratic Missouri congressman in the key early primary state, where he has been trailing in the polls.

Clyburn — a six-term congressman — has been courted by all nine Democratic candidates. He confirmed his long-expected choice of Gephardt late Tuesday.

“I won’t be coy about that,” he said, noting that he would formally endorse Gephardt at Allen University.

Clyburn said his decision wasn’t affected by the announcement earlier this week that former Vice President Al Gore, the party’s 2000 nominee, was endorsing the national Democratic front-runner, Howard Dean.

How much weight Clyburn’s endorsement will carry is an unknown, but because black voters could account for half of the votes in the partisan contest, the state’s pre-eminent black politician could play a pivotal role.

His formidable political organization — “the Jim Clyburn Network,” he calls it — extends through small-town city halls, churches, civic groups, Masonic temples, fraternities and sororities across his black-majority 6th Congressional District, which stretches from Columbia to Charleston.

His fish fry is an annual rite at S.C. State University’s homecoming game, where Clyburn renews ties with a web of fellow alumni of the historically black school. This week, he’s sending out 3,559 Christmas calendars to supporters, a gauge of his political reach.

“It’s hard not to do what Jim Clyburn says,” said Columbia City Councilman E.W. Cromartie II. As the state’s only black congressman since George Washington Murray in 1897, Clyburn, 63, also carries weight statewide.