WASHINGTON — South Carolina Sen. Lindsey Graham became the first GOP presidential dropout to endorse a former rival Friday — and he chose Jeb Bush over Senate colleagues Marco Rubio, Ted Cruz and Rand Paul.

“Last night [at the GOP debate] I heard from Jeb Bush the right answers,” Graham said, ­explaining his decision with Bush at his side.

“He demonstrated somebody in my view who is ready on Day One to be commander in chief.”

Bush can use all the help he can get. He’s at about 5 percent in recent polls.