"We've seen this before ... [and] it doesn't end well."
This is her 15th national union endorsement, compared to two for Bernie Sanders.
The featherweight ripped Trump for weighing in on his sport.
Rep. Keith Ellison (D-Minn.) says that Trump’s rhetoric could provoke violence.
Julian Hattem 11/24/15 10:35 AM EST
"Would I approve waterboarding? You bet your ass I would," he said Monday.
The new ad will air in Iowa and New Hampshire.
Carson, who had been in first place last month, fell to third in the latest Quinnipiac poll.
Those polled also favor sending American ground troops to fight ISIS.
Trump lambasted the media for not showing the entirety of the crowds watching his speeches.