Former Vice President Joe Biden began October with about $9 million in his campaign account—less than half the available cash of three other top-tier Democratic presidential contenders, according to Federal Election Commission reports filed late Tuesday.

Mr. Biden’s campaign funds lag behind Vermont Sen. Bernie Sanders’s nearly $34 million, Massachusetts Sen. Elizabeth Warren’s nearly $26 million and South Bend, Ind., Mayor Pete Buttigieg’s more than $23 million, the reports show.

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