Clint Eastwood is back on the Republican stage. But this time there’s a script, a 30-second time limit and none of the potential tripwires of live television.

Mr. Eastwood, whose long and sometimes incoherent monologue at the Republican National Convention in August left many ardent Republicans cheering but others dumbfounded, is the star of a new commercial from the “super PAC” American Crossroads.

But if he was all jokes in Tampa, Fla., Mr. Eastwood is nothing but serious in this new advertisement, in which he indicts President Obama’s term as a failure and urges people to vote for Mitt Romney.

“Obama’s second term would be a rerun of the first, and our country just couldn’t survive that,” he says. “We need someone who could turn it around fast, and that man is Mitt Romney. There’s not much time left, and the future of our country is at stake.”