On May 10, Public Policy Polling released this general election poll for president, including the Libertarian and Green likely nominees. The results: with Hillary Clinton as the Democratic nominee, Clinton 42%, Trump 38%, Gary Johnson 4%, Jill Stein 2%, other or undecided 13%.
With Bernie Sanders as the Democratic nominee, Sanders 47%, Trump 37%, Johnson 3%, Stein 1%, other or undecided 12%. Thanks to Michael for the link.