Undaunted, the freshman GOP senator posted an online video in which he read aloud a litany of "mean tweets" about himself. [...] Sasse again finds himself tangling with Trump and his supporters after he became the first Republican senator to declare publicly he would not back Trump if the billionaire businessman wins the GOP nomination for president. A Nebraska state senator called Sasse's comments "very immature," while Trump dismissed the suggestion that conservatives should consider backing a third-party candidate. "The vast majority of people supporting Donald Trump are well-meaning people who are angry about the direction of the country and feel we in Washington aren't urgent about solving the problems," said Sasse, a former college president who was elected in 2014 with support from the tea party. McConnell, now the Senate majority leader, took umbrage at Sasse's comments questioning taxpayer subsidies for health care for members of Congress. Sasse answered his own question by saying the Senate's purpose is to shield lawmakers from "an obsession with short-term popularity so we can focus on the biggest long-term challenges we face." If Trump is the party's nominee, "my expectation is that I'll look for some 3rd candidate — a conservative option, a constitutionalist," Sasse wrote on Twitter.