Facing a prospective tab of more than $1 billion to finance a general-election run for the White House, Donald Trump reversed course Wednesday and said he would actively raise money to ensure his campaign has the resources to compete with Hillary Clinton’s fundraising juggernaut.

His campaign also is beginning to work with the Republican National Committee to set up a joint fundraising committee after his last two rivals—Texas Sen. Ted Cruz and Ohio Gov. John Kasich—dropped out in the wake of Mr. Trump’s resounding Indiana win on Tuesday.

“I’ll be putting up money, but won’t be completely self-funding,” the presumptive Republican nominee said in an interview Wednesday. Mr. Trump, who had largely self-financed his successful primary run, added that he would create a “world-class finance organization.” The campaign will tap his expansive personal Rolodex and a new base of supporters who aren’t on party rolls, two Trump advisers said.

The new plan represents a shift for Mr. Trump, who has for months portrayed his Republican opponents as “puppets” for relying on super PACs and taking contributions from wealthy donors that he said came with strings attached.

Mr. Trump’s creation of a joint fundraising committee comes eight months behind that of his likely general-election foe, Mrs. Clinton. She and the Democratic National Committee reached an agreement last August to create the Hillary Victory Fund, which raised more than $60 million through the end of March. Of that, about $13 million has been transferred to Mrs. Clinton’s campaign, while nearly $6 million has gone to the DNC.