Republican candidates for the U.S. Senate in New Hampshire, Pennsylvania, North Carolina and a few other key states are struggling under the weight of Donald Trump’s candidacy. But there seems to be at least one exception: Republican Sen. Rob Portman of Ohio.Polling released in the past few days — including a Monmouth University survey of Ohio released on Monday — confirms that Portman is not being swamped by the anti-GOP sentiment Trump is stirring up.