Two newcomers, Senators Rand Paul of Kentucky and Ted Cruz of Texas, are the hottest figures in the dominant conservative wing of the Republican Party. In markedly different ways, they both claim to be the heirs to the party’s contemporary patron saint, Ronald Reagan. In a short time, these first-term lawmakers have electrified elements of the party’s base and rattled the Senate establishment. The top two Republican leaders in the chamber, Mitch McConnell of Kentucky and John Cornyn of Texas, have had to defer to the political popularity of their junior colleagues. Both are outside the party mainstream, rock stars...