In the wake of the Mueller report, a new CNN poll shows an increase among Democrats for Donald Trump's impeachment while Republicans remain adamant in their support of the president, evidence of what presidential historian Richard Striner describes as tribalism -- fierce political loyalty beyond ideology and mere partisanship.Meanwhile, former vice-president Joe Biden is the current front-runner among Democrats seeking their party's nomination. But Biden's status is starting to take a hit from the party's progressive wing.On the show: Mileah Kromer is associate professor of political science and the director of the Sarah T. Hughes Field Politics Center at Goucher College; she oversees the Goucher Poll. Richard Striner is a professor of history at Washington College and an author of books on American presidents, film and architecture.