Congressional candidates who speak of “liberating the American underclass” are flush with campaign donations. The likely Democratic presidential nominee has not only moved to the left on a range of issues, but now routinely rails against the influence of “big money.”

There are plenty of signs that Senator Bernie Sanders of Vermont has left a mark on the political moment. But some liberal Democrats are beginning to worry that as Mr. Sanders continues his quest for the nomination, his chance to build a lasting legacy may be slipping away.

Liberals who remember watching with fear and awe how the Christian Coalition rose from the ashes of Pat Robertson’s 1988 campaign — using his mailing lists and leftover cash to build a grass-roots organization, publish conservative voter guides and promote candidates at every level — are asking when, or if, Mr. Sanders will help do the same for the left.

Far from laying the foundation to transform his campaign into a movement, Mr. Sanders is wrapped up in the race itself, sharpening his attacks on Hillary Clinton and demanding she debate him before the June 7 primary in California. And many of his supporters are following his cue.