Anywhere with a lot of Millennials and/or African Americans.
North Carolina, Virginia, Colorado, would be the most likely places, but also Cleveland/Cincinnati, Detroit, Milwaukee, Madison, Iowa City, Des Moines, Las Vegas, Orlando, Tampa, Jacksonville, Miami-Dade... Atlanta/Phoenix if feeling frisky.
This. President Obama could visit the Southern states of Georgia, Florida, and North Carolina. He could forward his message for AA voters in states like Mississippi and South Carolina (Even though they likely won't be competitive until maybe the 2030s) as well.
Throughout the West, Arizona and Colorado should be targeted in my opinion.
In the Midwest, Iowa is slipping, so that should be priority while also visiting Wisconsin and maybe Minnesota.
Up in the Northeast, there isn't much to say except New Hampshire (due to the senate race) and Pennsylvania.
