The next round of Republican primaries and caucuses on Saturday could put to rest an emerging theory that could blunt front-runner Donald Trump’s air of inevitability — that he struggles in contests where only Republicans can vote.

That is, when Democrats and independents can cross over and vote in a GOP primary, Trump gains because of his appear to working class Democrats harmed by the economy. While that may help him in the general election, some think the lack of Democrats in closed primaries suggests his support among Republicans is overrated.

Trump has blasted through the Republican primary calendar, racking up victory after victory, accumulating the most delegates and making himself the clear front-runner for the nomination for president. But three of the states he has lost — Oklahoma, Alaska and Iowa — allow only Republicans to participate.

That has led some supporters of Sen. Ted Cruz to argue that Trump can be overcome when the primary seasons turns to a greater number of states with closed primaries and caucuses.

“The big loser last night, contrary to what you’re seeing everywhere else, was Donald Trump,” Rep. Mo Brooks, R-Ala., said on MSNBC following Super Tuesday. “Now, maybe he doesn’t realize it yet and maybe people in the mainstream media don’t realize it yet … Donald Trump’s Achilles’ heel was badly exposed last night, and I think you’re going to see that play out over the next few weeks.”

Saturday will offer evidence one way or the other. Caucuses in Nebraska, Maine and Kentucky, as well as the Louisiana primary, are all closed affairs.

“There’s not doubt that Trump has done better in primaries and caucuses when independents can vote,” said Stephen Voss, a University of Kentucky political science professor. “That’s likely to only get worse as Republican spokespersons and leaders try to convince voters not to vote for Trump. He’s done worse (in Republican-only contests) than he would otherwise. The flip side is that worse than he would have done may still be good enough to win.”

Polling is notoriously unreliable in predicting the outcome of low-turnout caucuses, but the signs are good for Trump. A Western Kentucky University poll released last month gave him a 13-percentage point lead over Sen. Marco Rubio in the Bluegrass State. Voss said Trump recently drew a huge crowd in Louisville. He added that Rubio, meanwhile, canceled a rally that had been planned for Friday.

A pair of surveys conducted this week in Louisiana showed Trump with double-digit leads over Cruz.

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Josh Putnam, a University of Georgia political scientist, said Trump is in a similar position to Mitt Romney at a comparable stage of the 2012 primary season.

“The only thing we’ve really done is take Romney’s name out and put Trump’s name in,” he said. “It is really a war of attrition, and (Trump’s critics) may find out soon that resistance is futile.”

If Trump does stumble on Saturday, it will encourage opponents who are holding out hope that he still can be stopped. A number of big states have closed primaries, including winner-take-all Florida, which will make or break Rubio on March. 15.

Putnam noted that Trump easily won the Republican-only Nevada caucuses. He said it is premature to conclude the real estate mogul, based mostly on low-turnout caucuses, will struggle to win without independents.

“I’d like to see more data before I put much stock in that,” he said. “I don’t think we’ve had a good test of that.”