OSKALOOSA, Iowa—Donald Trump said there was no cap in the amount of money he would be willing to put into his campaign for the Republican nomination for president if his message continues to resonate with voters. Speaking at a free-wheeling press conference following a rally in this city east of Des Moines, Mr. Trump also attacked Republican rivals Jeb Bush and Scott Walker and Democrat Hillary Clinton with accusations that they were beholden to wealthy donors. Mr. Trump, a real-estate developer and television personality, has almost entirely self-financed his campaign so far. “Bush is controlled by those people. Walker is...