HOUSTON — Candidates for the Democratic presidential nomination are sprinting from coast to coast in search of campaign donations over the next 18 days, moving urgently to stockpile cash for their big fall push — and to avoid a death spiral that a weak third-quarter fundraising tally might prompt.

With the third debate behind them — the fourth is in mid-October, well after the fundraising deadline — the candidates are devoting themselves to finishing with a flourish what is typically a difficult three-month fundraising period, according to a review of invitations and interviews with officials and donors.

Former vice president Joe Biden’s campaign has tentatively arranged at least 16 post-debate fundraisers before the Sept. 30 deadline, including one here Friday and another in Dallas on Saturday. Sen. Kamala D. Harris (Calif.) has at least 17 planned. Pete Buttigieg, the mayor of South Bend, Ind., was scheduled to hold at least 10, including one in Dallas on Friday