Ted Cruz, in his outsiders bid for the White House, has depended heavily on the largesse of just three wealthy donors to establish credibility and stay afloat amid a chaotic nominating process that killed off most of his rivals. Now, at perhaps the most desperate moment in his quest to win the Republican nomination, Cruz is learning the perils of relying on strong-willed magnates who carry their own agendas and have demanded an unprecedented level of control in how their money is spent.