“I have been a conservative Republican my entire life. But the Republican party as a whole has gotten so far out of touch with the American people,” says Bryan Brown, a 47-year-old Oregon resident. “I switched my registration so that I could vote for Sanders in the primary, but the day the primary is over I’m going to register as an Independent.”

Anger and alienation have turned conventional wisdom upside down in this presidential election. Self-styled outsider candidates like Donald Trump and Ben Carson have surged in the polls. And as Republican candidates debate their conservative credentials, support for Sanders shows how difficult it can be to pin down what exactly it means to be conservative.

“Once you get out of Washington ‘conservative’ can mean all sorts of different things. Voters are often left of center on some issues and right of center on others. So someone like Trump or Sanders who talks about themselves in a way that doesn’t fit into a pre-ordained box could be appealing to a lot of people,” says Chris Ellis, a political science professor at Bucknell University.

In some cases, longtime Republican voters who have decided to support Sanders, like MacMillan, are rethinking their political affiliation entirely. (“I’m inclined to say I might stay with the Democratic Party because the Republican Party has changed and it’s not the way it used to be,” MacMillan says.) Far from claiming to have experienced a political conversion, other Republicans argue that Sanders actually embodies conservative values.

“When I think of true conservative values I think of Teddy Roosevelt who earned a reputation as a trust-buster,” says Jeff DeFelice, a 38-year-old registered Republican voter living in Florida. “Now look at Bernie. He’s the only one willing to stand up to the big banks. The big banks control an obscene amount of wealth in this country and he wants to go after them.” If Sanders looks like “a viable candidate” by the time the primary rolls around, DeFelice says he’ll switch his party affiliation to vote for the senator.

Sanders’s promise to wrest power away from Wall Street and return it to the American middle class taps into the same vein of populist anger that fueled the rise of the Tea Party. It’s also a message that resonates with mainstream Republicans and Democrats. Sixty-two percent of Republicans, for example, believe that large corporations wield too much influence on American politics, according to a New York Times/CBS News poll conducted in May.

“Sanders has focused primarily on economic issues on which Americans are not divided,” says Elizabeth Coggins, a professor at Colorado College who studies American political psychology and ideological identification. “There is a strong consensus in agreement with Sanders on many of his core ideas, and his rhetoric has been largely centered on these sorts of issues.”