“ I ask all of you to join me in working as hard for Barack Obama as you have for me,” Mrs. Clinton said. “I have served in the Senate with him for four years. I have been in this campaign with him for 16 months. I have stood on the stage and gone toe-to-toe with him in 22 debates. I’ve had a front-row seat to his candidacy, and I have seen his strength and determination, his grace and his grit.