Brexit
"When I speak to young people, I implore them, and I implore you, to reject those calls to pull back."
The president said Britain's EU membership has helped make the world freer, richer and better able to tackle everything from Russian aggression to terrorism.
WHAT'S HAPPENING
"A Brexit could do severe regional and global damage by disrupting established trading relationships."
With hashtags, naturally: #PleaseDontGoUK #HugABrit
Most opinion polls show voters are split.
"I want a better deal for the people of this country to save them money and to take back control," Johnson said.
Half of Brits want to stay in the European Union, while the rest want out.