CHARLOTTE, N.C. — Former President Bill Clinton and President Obama hugged onstage Wednesday night after Mr. Clinton delivered an impassioned plea on behalf of Mr. Obama’s re-election, the 42nd president nominating the 44th to a second term with a forceful and spirited argument that Democratic values would restore the promise of the middle class.

The former president delivered a point-by-point rebuttal of the arguments made during the Republican National Convention last week, warning against Republicans taking back the White House and declaring, “We can’t let it happen.”

He offered an equally detailed affirmative case for the re-election of Mr. Obama, saying there was no question the country was in a better position than it was four years ago.

“We simply cannot afford to give the reins of government to someone who will double down on trickle down,” Mr. Clinton said, repeatedly bringing the crowd at the Democratic convention to its feet. He added, “I love our country so much and I know we’re coming back.”