It's rare for a populist candidate to go far in presidential politics. It's almost unheard of for two of them to advance as far, in the same year, as Donald Trump and Bernie Sanders have. Trump remains the frontrunner for the Republican nomination. Sanders still has a shot at the Democratic one. Their successes share some common themes, which are fueling speculation that populist voters from the left and right might come together to back a single candidate come November, or even form their own political party in the years to come.

The idea is that anger at Wall Street, stagnating middle-class wages, international trade agreements, the influence of rich donors in political campaigns and, most of all, American political elites, will bring millions of blue-collar voters into an unlikely alliance.

Perhaps they will support Trump against Hillary Clinton in the fall, or perhaps, as the liberal former Labor Secretary Robert Reich spelled out last week in an op-ed, they will revolt against both parties in the next presidential race.

Reich envisions a 2020 election where a so-called "People's Party" emerges with a platform blasting "crony capitalism," breaking up banks and soaking the rich to send more money to workers from the government. "Millions who called themselves conservatives and Tea Partiers joined with millions who called themselves liberals and progressives," he writes, "against a political establishment that had shown itself incapable of hearing what they had been demanding for years."

That's possible; America saw a semi-related party, Ross Perot's Reform Party, burst onto the scene in the 1990s, though it never won a seat in Congress or an electoral vote for president. Perot ran on opposition to expanded trade, an area where exit polls suggest Trump and Sanders supporters agree: In Michigan, two-thirds of Trump voters, and nearly as many Sanders voters, told exit pollsters that trade kills U.S. jobs. The share was similar for Trump in Ohio, and still a majority for Sanders.

But other polling suggests an alliance of today's Trump supporters and today's Sanders supporters remains unlikely, this fall or in the future.

While they agree on key economic issues, Trump and Sanders voters disagree on a lot of others - and they don't care much for each others' standard bearer. They're also very different, demographically.

Polling shows little evidence that Trump has a shot to win large pockets of Sanders voters in November, should Clinton maintain her lead and win the Democratic nomination. Among Democratic-leaning voters who want prefer Sanders to win the party nod, only 13 percent have a favorable view of Trump, compared to 86 unfavorable, according to a Washington Post-ABC News national poll earlier this month.

In a head-to-head match-up of Clinton versus Trump, 10 percent of Sanders backers say they’d support Trump, 77 percent Clinton, and the rest say they would vote for neither or not vote at all. That’s not a great conversion rate for Clinton from Sanders backers, but it’s slightly better than Trump’s 67 percent support among voters who preferred other Republican candidates.

We obviously don't have polling on a potential consensus candidate for the Sanders/Trump crowds in 2020, but any such candidate would have to unify two very different types of populist voters.

Post-ABC polling from this month shows Sanders supporters tend to be young, while Trump supporters tend to be in or nearing retirement. Trump draws mostly from non-college grads; Sanders draws equally across the education spectrum. Most importantly, Trump supporters largely identify as “conservative” while Sanders tend to identify as “liberal”.

While both men draw more than 40 percent of their support from voters earning $50,000 a year or less, Trump's supporters are more likely to say they are struggling to stay in their current economic class than Sanders' are (41 percent vs. 28 percent). And Trump supporters' economic anxieties entwine with racial and immigration anxieties in a way that they do not for Sanders supporters.

Nearly 8 in 10 Trump supporters favor deporting all immigrants who entered the country illegally, compared to fewer than 2 in 10 Sanders supporters. A similarly small share of Sanders backers favoring temporarily banning Muslim entrants to the United States, compared to 7 in 10 for Trump.

More than half of Trump supporters (54 percent) say whites losing out because of preferences for blacks and Hispanics is a bigger problem in the country than discrimination against those minority groups. Only 15 percent of Sanders supporters agree.

You can see the rub, then, in Reich's vision for the People's Party platform. It would, he predicts, call for "getting big money out of politics, ending 'crony capitalism,' abolishing corporate welfare, stopping the revolving door between government and the private sector, and busting up the big Wall Street banks and corporate monopolies" - and also push to revoke the Trans-Pacific Partnership trade agreement, "hike taxes on the rich to pay for a wage subsidy (a vastly expanded Earned Income Tax Credit) for everyone earning below the median, and raise taxes on corporations that outsource jobs abroad or pay their executives more than 100 times the pay of typical Americans."

That's a Sanders platform, basically. And while large swaths of it would seem to appeal to Trump populists, it remains silent on some of those voters' most galvanizing issues.

Unless the Sanders coalition is willing to reverse course on deportation, or Trump voters suddenly stop caring about immigration or racial preferences, it's hard to see these voters uniting to disrupt American politics, this year or any other.

Emily Guskin contributed to this report.

The Post-ABC poll was conducted March 3 to 6 among a random national sample of 1,000 adults reached on land-line and cellular phones. The analysis above is based on interviews with 150 Democratic-leaning voters who support Bernie Sanders and 136 Republican-leaning voters who support Trump for their party's nomination; the margin of sampling error for each group is plus or minus 9 and 9.5 percentage points, respectively. 

U.S. presidential candidate Bernie Sanders rallies supporters in Seattle, Washington ahead of that state's caucus. Rough Cut (no reporter narration). (Reuters)