The prospect of a Donald J. Trump nomination is accomplishing what a diverse and talented field of Republican presidential contenders could not: uniting the party’s big-money donor establishment.

Some of the wealthiest conservative givers in the country are helping pay for a series of last-ditch attacks to wound Mr. Trump, disclosures filed on Sunday night revealed, even after previously backing rival Republican candidates. And officials involved with the political groups have made clear that they are aggressively raising more money to fight Mr. Trump, hoping to deprive him of enough delegates to win the Republican nomination outright. That would set the stage for a contested convention in July.

Our Principles PAC, a group set up to highlight Mr. Trump’s past liberal positions, took in $4.8 million last month, with a roster of donors that shows it has significantly expanded beyond the Ricketts family, which provided the group’s early funding.

Beyond the Rickettses, who collectively provided another $2 million to the group, Our Principles raised $500,000 from William E. Oberndorf, a California investor who had previously backed Jeb Bush, and $100,000 from Harlan Crow, a Texas real estate developer, who had supported Senator Marco Rubio of Florida. Our Principles also raised $1 million from Warren Stephens, an Arkansas investment banker who gave large checks to “super PACs” backing four candidates who have since quit the race.