One of the wealthiest and most influential Republican donors in the country is throwing his support to Senator Marco Rubio of Florida, a decision that could swing millions of dollars in contributions behind Mr. Rubio. The decision by the donor, Paul Singer, a billionaire New York investor, is a signal victory for Mr. Rubio in his battle with his rival Jeb Bush for the affections of major Republican patrons and the party’s business wing. It comes as a major blow to Mr. Bush, who is seeing his once vigorous campaign imperiled by doubts among supporters, and whose early dominance of...