Donald Trump keeps saying the system is rigged against Bernie Sanders. Here's why.

For months Donald Trump has said "Crooked Hillary Clinton" has given Bernie Sanders a raw deal.

Now the presumptive Republican nominee who once lambasted Sanders for being "beyond socialism" has turned his softer tone with the independent senator from Vermont into a direct appeal, telling Sanders supporters they have a home with him Tuesday night.

"To all of those Bernie Sanders voters who have been left out in the cold by a rigged system of super delegates, we welcome you with open arms," Trump said during his victory speech. "And by the way, the terrible trade deals that Bernie was so vehemently against - and he's right on that - will be taken care of far better than anyone ever thought possible."

It's the continuation of a strategy Trump began to more heavily adopt in April.

"The Democrats have treated Bernie very badly, and frankly he should run independent," Trump said after winning all five Northeastern primaries.

Then after Sanders won the Indiana primary, Trump took to Twitter nearly congratulating the progressive Democrat for beating Clinton in the "rigged" system working against him.

Trump's strategy was clear from the start: If Sanders won't run as an independent, as he's already said he won't, then Trump wants Sanders's supporters to vote for him over Clinton in the general election.

"This is probably tactically smart and might pick up a certain amount of goodwill at the moment," and at zero cost to Trump — all he has to do is put the idea out there, said Jedediah Purdy, a Duke University law professor who's written extensively on American political identity.

And on one hand, it's not that far-fetched an idea. Sanders and Trump have ignited two varyingly successful populist movements this presidential election season, bolstering their campaigns with voters' rampant discontent with the status quo. With Clinton's nearly insurmountable lead in the Democratic race, Trump wants to unify his "political revolution" with Sanders's.

There are some of these voters out there: Traveling to campaign rallies in New Hampshire and South Carolina earlier on, Matt Dickinson, a political science professor at Middlebury College, said he was "shocked" by how many voters said they were going to vote for either Sanders or Trump.

But come November, when voters will most plausibly be choosing between Trump and Clinton, likely few feeling the Bern will want to make America great again.

Why? Not every populist movement is the same, especially when it comes to voters supporting a staunch progressive and those supporting Trump.

The Trump-Sanders comparison doesn't really make sense

On the surface, the comparison is a simple one: "There is a distinctive anti-establishment tenor to the campaigns," Dickinson said. Since the 2000s, the last substantive period of economic prosperity, income has stayed relatively stagnant, which has led to a lot of "economic dissatisfaction," Dickinson said.

It's clear both Trump and Sanders have been able to tap into that basic economic frustration. Paired with their attacks on 30 years of trade deals ruining American prosperity — "which is not a trivial rebellion and does cross the parties," Purdy said — the Trump-Sanders likeness is formed.

Voters are disenchanted with money in politics; they see a rigged system playing into Washington's inner circle, and where Clinton has wrapped herself in Obama's mantle, Sanders and Trump are ostensibly promising change.

But the changes they are selling are drastically different. FiveThirtyEight's Nate Silver put it this way:

Trump and Sanders are fundamentally different breeds of candidates who are situated very differently in their respective nomination races.

You can call both "outsiders." But if you’re a Democrat, Sanders is your eccentric uncle: He has his own quirks, but he’s part of the family. If you’re a Republican, Trump is as familial as the vacuum salesman knocking on your door.

In terms of ideology, Dickinson said the difference between Trump and Sanders's political revolutions is how their supporters want to see change, and as foreign and "vacuum salesman"–like Trump may seem to the establishment, his voters want to reestablish conservative and traditional values.

"Trump and Sanders supporters think the current system too often empowers the privileged — the difference is how they propose to redress those inequities," Dickinson said. "At the risk of simplification, I think Sanders’s supporters believe in more government regulation and a bigger state capacity: Let’s break up banks, let’s institute single-payer health care, let’s offer free tuition to public schools, let's raise corporate taxes.

"Trump’s people share those concerns, but they just want the economic system to operate on a level playing field, without the government necessarily playing favorites. This means smart trade and reducing overseas labor competition, among other things."

Turning ideology to policy, the camps are split. The Republican Party remains the party of wealth, and Trump's tax plan, which proposes substantial tax cuts for the rich and the poor, is emblematic of that, Purdy said.

Once those policies are set in the general election, sparing massive realignment, Trump "won't have the kind of overlap that might seem to be of prospect in the primaries," Purdy said.

Most Bernie supporters will probably fall in line with the Democratic Party

Recent polling from CNN found that Sanders voters overwhelmingly support Clinton over Trump, at an 86-to-10 percent margin.

"The reality of staring at Donald Trump over there as the Republican nominee, now that that's settled, I think will do wonders to concentrate the minds of Democrats," Abramowitz, a political scientist, told Vox's Jeff Stein.

While it's still early, the margin of crossover voters from Sanders to Trump is not far from 2008, when CBS News Election survey found 16 percent of John McCain voters would have supported Clinton if she had been the candidate – of those voters, 43 percent identified as independents.

Sanders has performed best in primaries allowing independent voters to participate. In New Hampshire, he won independents by almost 50 percentage points while only winning Democrats by 4 percentage points, and again in Ohio, where he won 66 percent of independents but only 35 percent of Democrats.

But Sanders's independent voters — young voters who don't have established ties and upper-echelon young professionals who consider themselves progressives — are different from Trump independents, whom Dickinson finds to be "mostly downscale disaffected voters that are more generally moderate and could swing between the parties."

Independents are not all moderate, nor are they nonpartisan. And as political scientists Samara Klar and Yanna Krupnikov, who authored a book on independent politics, wrote for Vox in January:

People’s preference for one party over the other can lead them to dismiss messages from the opposing party no matter how compelling or well-thought out these messages may be.

Once it comes to a general election, if the choice is Clinton, stay home, or Trump, "a lot of Sanders supporters will hold their nose and vote for Clinton. She has gone closer to [Sanders] than Trump has," Dickinson said.

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