The Democratic Party’s new directive that candidates must have at least 130,000 donors to qualify for the third primary debate in September arrived virtually without warning on Wednesday morning, and immediately sent shock waves through presidential campaigns worried that it would distort their priorities and affect the way they operate.

Two-thirds of the sprawling field of 23 candidates are probably at risk of falling short of that threshold, and news of the more stringent rules set off a flurry of frustrated early-morning text messages, emails, calls and meetings as campaigns reassessed the path forward, according to multiple 2020 campaign officials.

While the Democratic National Committee had long intimated it would raise the bar to qualify for later debates, many 2020 strategists were stunned by the 130,000-donor threshold, which doubles the requirement for the first two debates in June and July and which few are close to hitting. Some candidates questioned whether the party’s new donor threshold would winnow the field too severely, before most voters even tune in to the race.

Most declined to discuss their frustration with the D.N.C.’s rules on the record or to indicate how exactly they would shift tactics, saying their campaign plans were confidential. But campaign after campaign said the party’s donor requirements are skewing the way they allocate resources, forcing them to choose between investing in staff or pouring more money into ads on sites like Facebook, where prices are soaring to dizzying new heights. Two campaigns said digital vendors are currently quoting them prices of $40 and up to acquire a new $1 donor.