Sean Spicer, the committee’s chief strategist, acknowledged this week that the committee had begun informing state parties and statewide campaigns that fulfilling such plans would now be “slower.” He said the pledges had been made with the assumption that Republicans would have “a presumptive presidential nominee by now.”

Just as revealing, the party is also taking steps to create a separate fund-raising entity dedicated to Senate races, an acknowledgment that many of the wealthiest contributors are increasingly focused on protecting Republican control of Congress rather than on a presidential campaign they fear is lost.

Taken together, the party’s financial difficulties illustrate the considerable fallout Republicans are facing from a nominating contest that could last through mid-July and that features two leading candidates, Donald J. Trump and Senator Ted Cruz, who are deeply troublesome to many leading Republican donors.

“I think everything is up in the air,” said Matt Borges, the chairman of the Republican Party in Ohio, which in addition to being a perennial, and perhaps the pre-eminent, swing state is also home to a competitive Senate race this year.