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SHELBY TOWNSHIP, Mich. – Gone were the petty torments about the size of Donald J. Trump’s hands and the orange hue of his skin. In fact, Senator Marco Rubio was all business on Wednesday, leaving aside the mean jokes about Mr. Trump.

Mr. Rubio spent the day in Michigan after a string of disappointing defeats in the Super Tuesday contests, rallying a rowdy but relatively small crowd compared with the ones he had been drawing.

He spoke to several hundred people at a banquet hall in Salem Township, Mich., in the late afternoon. And he kept the Trump insults limited to the developer’s business dealings and political ideology.

“If we choose Donald Trump as our nominee, we will lose in November,” he said. “If we choose Donald Trump as our nominee, we will put in charge of the conservative movement someone who is not a true conservative. And if we choose Donald Trump as our nominee, he will have carried out the most elaborate con job in the history of American politics.”

Mr. Trump, he added, was exploiting fear. And he urged people not to be taken in.

“We are at our worst when we appeal to those fears and to anger and frustration as the base of our movement,” he said, drawing nods and affirming shouts from the crowd. “Should we allow them to motivate us? Yes. Should we allow them to define us? Absolutely not.”