WASHINGTON—Donald Trump’s pick to run the Drug Enforcement Administration withdrew and his selection for defense secretary faced growing skepticism from some GOP senators, marking the latest challenges to the president-elect’s efforts to staff his second term’s top positions. On Tuesday, the DEA pick, Chad Chronister, a Florida county sheriff, said that the “gravity of this very important responsibility set in” and that he concluded he should pull his name. He had come under attack from conservatives for a 2020 arrest of a pastor who defied Covid-19 restrictions and held a large church service. Meanwhile, Pete Hegseth, Trump’s pick to run...