Complex sentence: Morrow first backed U.S. Senator Ted Cruz of Texas, who withdrew from the race for the 2016 presidential nomnation but then switched to party frontrunner Donald Trump of New York City, for whom Stone had earlier been an advisor and still supports.
Split into simpler sentences: Morrow first backed U.S. Senator Ted Cruz of Texas for the 2016 nomination. When Cruz withdrew from the race, Morrow switched his support to party frontrunner Donald Trump of New York City, for whom Stone had earlier been an advisor and still supports.