But as the race continues and Sanders earns more exposure, his standing may change, said University of Wisconsin-Madison political science professor Barry Burden.

"He really has not taken much fire yet," Burden said. "Clinton has been campaigning against him, she has been pointing out his inconsistencies, or maybe lack of loyalty to Obama ... but those aren't things that really bother independent voters much, or Republicans — and he's taken no fire from Republicans, yet."

Burden said Sanders may also be seeing a bump from Republican-leaning voters who are dissatisfied with their party's frontrunners.

"At least a small share of them would be willing to think about a Sanders candidacy at this point," Burden said. "I suspect many of those would come home to the party in the general election."

But should Trump become the GOP nominee, it's hard to imagine him carrying the mantle of the party in the general election, Burden said. Not only does he have a net negative rating among all voters, but within his own party, he has a net favorability of zero. Among Republican candidates, it appears Rubio has the most room to morph among Wisconsin voters, he said.

It's still far too early to predict anything with much certainty, Franklin said — so much that happens between now and November will determine things like turnout, independents' support and enthusiasm.

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