Rep. James Clyburn’s endorsement of former Vice President Joe Biden for president is a big deal. It could be the most consequential endorsement since Ted Kennedy endorsed Barack Obama against Hillary Clinton in 2008.

Conventional wisdom in the last quarter century or so has gone from holding that political endorsements matter a lot to holding that they matter not at all. The truth is in between: Endorsements don’t matter, except when they do. That of Clyburn, the longtime congressman from South Carolina and the highest-ranking black elected official in either party, is one that matters so much that it could hand the Democratic nomination, and eventually the presidency, to Biden.

It is obvious that Biden’s campaign has been reeling all month. I was among those who, perhaps prematurely, already pronounced it dead. But that was before socialist Bernie Sanders became the race’s clear front-runner with none of the other candidates having established themselves as the obviously leading alternative. With the Democratic establishment reportedly panicking about a massive, partywide defeat if the Castro-loving Sanders leads the Democratic ticket, they desperately needed somebody to give voters a signal as to which non-Sanders candidate to rally around.

Today, Clyburn gave that signal.

Clyburn is particularly powerful among South Carolina Democrats because he is particularly beloved. Respected by virtually all the state’s Democrats, he is especially treasured by black voters who make up some 60% of South Carolina’s Democratic electorate. Clyburn endorsed eventual nominee John Kerry in 2004 and helped Clinton defeat Sanders in 2016.

The South Carolina primary is Saturday, Feb. 29. With Biden protecting what had been a big lead, but now a shrinking one, in the Palmetto State, Clyburn’s endorsement the day after Biden showed some spunk in the latest televised debate could well provide Biden a substantial margin of victory rather than a nail-biter. If that happens, it could give Biden the necessary momentum to do well in many of the 17 states or territories that vote just three days later – especially among the black voters who predominate in the five southern states that vote March 3.

Sanders clearly has enough of a grassroots organization to rack up plurality victories in plenty of states if several opponents continue to divide the non-Sanders vote. But if Biden becomes the last candidate seriously standing between Sanders and the nomination, Biden still could prevail.

Clyburn’s endorsement makes it much, much more likely that Biden could become that last person standing. For Democrats worried about an anti-socialist wave in November, the consolidation behind Biden might happen just in the nick of time.