The forum is the first since Donald Trump scored multi-state wins on Super Tuesday.
The retired neurosurgeon failed to win any state that voted on Super Tuesday.
The TV presenter says the Republican frontrunner would be more moderate in office.
In his own White House-style event, Donald Trump paid lip service to unity while taking shots at his party's leaders.
Overall, just 48 percent of adults in the Super Tuesday states accept that climate change is man made.
March marks the second month in the presidential election's primary season.
Several candidates' home states, including Vermont and Texas, are voting in Super Tuesday.