“A lot of people think that in that crowded field, he could break out,” said Senate Majority Whip John Thune (R-S.D.).

He added: “If we can run a race against a person that’s an out-of-the-closet socialist and promoting socialist ideas, it’s a great contrast for us.”

The strategy shows Republicans are much more comfortable talking about Sanders and tying other Democrats to his brand of socialism than they are in defending this year’s meager legislative agenda. But Republicans could be making the same mistake Democrats made four years ago, when Trump launched his presidential campaign and they began salivating over the prospect of a Senate sweep.

That misunderstanding of Trump’s appeal is now the subject of repeated examination by Democratic politicians and strategists.