But the main thrust of his argument was more expansive, touching on the political left's existential concerns over what they see as hypocrisy in Trump and disdain for democratic norms.
"They know none of this is popular", he added.
"He might be a nice guy", Trump said of Tester, "except he tried to destroy Adm. Ronny Jackson, who is the most clean-cut, wonderful person that you ever met".
Obama joined gubernatorial candidate Andrew Gillum and Sen.
"When words stop meaning anything, when truth doesn't matter, when people can just lie with abandon, democracy can't work", Obama said.
Trump has tried to stoke up support for the Republicans by taking a hardline stance against a caravan of several thousand migrants heading toward the U.S. border with Mexico.
Knight said Trump was a "great president" in the mold of Harry S. Truman, and led the crowd in a chant of "Go get 'em, Donald". And Florida's governor and senate races are especially close.
Although he is not on the ballot, Trump's polarizing presence has loomed large over the campaign for Tuesday's midterm elections, and in many ways the USA political landscape reflects the deep racial divisions manifested recently in deadly attacks in Pittsburgh and Kentucky. "The consequences of any of us staying home really are more risky", Obama said.
Far from buddies, Trump has frequently blasted Obama's economic stewardship and handling of trade agreements while the former president has warned that the "character of our country is on the ballot".
Even as the campaign back-and-forth dominates the headlines, the beginning of the weekend also belonged to the grassroots liberal groups that have quietly been gearing up for a massive, final get-out-the-vote push. "Don't do that in front of them, come on", Obama calmly told one heckler. The stakes really are that high.
In West Virginia, Trump's third visit in three months was aimed at shoring up Patrick Morrisey, who is seeking to unseat Democratic Senator Joe Manchin. Gillum, the current mayor of Tallahassee, has 50 percent of support among likely voters, versus 46 percent for his GOP rival. That poll has an error margin or plus or minus 5 points, so make of that what you will, too. Like other Democrats in the state, he still expects Manchin to win.