The two progressive frontrunners, Sen. Bernie Sanders of Vermont and Sen. Elizabeth Warren of Massachusetts, will face off on the same debate stage on July 30 in Detroit, Michigan.

So far, the two candidates have shied away from directly taking shots at each other on the trail, but Warren's surge in the polls and fundraising represents a serious threat to Sanders' candidacy.

Some writers have argued Sanders views himself as the leader of an anti-establishment movement that serves as an adversary and challenge to the Democratic party apparatus, whereas Warren is more willing to work within it.

And as Politico reported earlier this month, a closer look at the Democratic primary polling we have so far indicates that the two candidates' current bases of support aren't completely interchangeable.

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20 Democratic presidential candidates are set to take the stage for two nights of Democratic primary debates in Detroit, Michigan on July 30 and 31. The two progressive frontrunners, Sen. Bernie Sanders of Vermont and Sen. Elizabeth Warren of Massachusetts, will face off on the same debate stage on July 30.

Both candidates will have the opportunity to showcase their different approaches to politics and policy in a quest to win over progressive-minded voters.

So far, the two candidates have shied away from directly taking shots at each other on the trail after reportedly promising each other back in December 2018 not to attack each other during the campaign.

While Warren has so far held up her end of the bargain, tensions are simmering between the two camps as Sanders' campaign has begun to publicly undercut Warren's commitment to building a truly progressive, anti-corporate movement, as well as her electability.

Read more: Democratic primary voters have lower expectations for Joe Biden and Bernie Sanders' next debate performance, but higher expectations for Kamala Harris

In March of 2019, Warren trailed Sanders by 20 points in Morning Consult's weekly Democratic primary survey, with 7% support compared to his 27%.

Since then, Warren's level of support has risen in Morning Consult to 14% while Sanders' has steadily fallen and leveled off at 18%, closing the gap between them to just 4 percentage points as of July 22, 2019, putting the Sanders campaign on notice.

Sanders' campaign has taken a few shots at Warren as her appeal surged

On June 19, Sanders' official Twitter account shared a Politico story with the headline, "Centrists are coming around to Elizabeth Warren as an alternative to Bernie Sanders" with the message: "The cat is out of the bag. The corporate wing of the Democratic Party is publicly 'anybody but Bernie,'" with Sanders later denying the tweet was about Warren.

As Warren has surged in the polls and closed the gap between her and Sanders, Sanders' surrogates have publicly refuted the narrative that Warren's increased support threatens Sanders' base. And in anonymous quotes to news outlets, at least one Sanders staffer cast doubt on her electability against President Donald Trump and questioned whether she could win.

Sanders, who has served in Congress since 1993, was the progressive standard-bearer in 2016 and drew a sharp contrast as an anti-establishment changemaker compared to the more establishment-aligned Hillary Clinton.

This time around, Sanders has reaped the benefits of heightened name recognition and a large, nationwide grassroots network of small donors from his 2016 run. This base allowed to the Vermont senator to rise close to the top of the field in most pre-primary polls and raise substantial cash from his dedicated supporters.

But Sanders' success in 2016 helped bring many of the policies he championed — including Medicare for All, free public college, and a $15/hour federal minimum wage — into the political mainstream. As a result, Sanders' progressive populism is no longer unique in the Democratic field.

This time around, he's not competing against just one world-famous mainstream candidate, but a mix of 20 progressive and moderate candidates, many of whom who have adopted his policy positions.

Read more: 'I've got a plan': Elizabeth Warren's running as the policy wonk candidate and her poll numbers keep going up

Meanwhile, Warren's 2020 campaign got off to a bit of a rocky start.

Following months of taunts from President Donald Trump, who called her "Pocahontas" and cast doubt on the validity of her Native American heritage, Warren took a DNA test, which backfired when she was widely criticized by Native American activists over the move.

Then, The New York Times reported that Warren's decision not to hold any private, closed-door fundraisers prompted her finance director to quit and lead many advisers to question whether her campaign was viable at all.

But Warren proved her critics and doubters wrong in the following months by leaning into her reputation as a policy wonk. She released a unique and detailed policy plan to address almost every imaginable issue from the cost of college to the opioid crisis, rolling out so many policies that "I've got a plan for that" became her unofficial slogan.

On the trail, Warren has won over support by illustrating her policy positions and overall message with compelling stories of her own life growing up in a working-class family in Oklahoma and putting herself through college and law school as a single mother and becoming one of the nation's leading experts on consumer finance law.

Her years of experience as a law professor have also enabled her to explain complicated concepts related to consumer protection, bankruptcy, and anti-trust law in an approachable and compelling manner to audiences, while Sanders' gruff personal style can sometimes come across as grating. One such example came in the first Democratic debate when Sanders accused moderator Rachel Maddow of mischaracterizing his position on guns when she was directly quoting him.

Furthermore, Sanders commitment to labor rights and justice has come under scrutiny amid a very tense and public labor dispute with the union representing his campaign staff, which eventually negotiated a deal for higher wages and fewer hours.

Read more: Bernie Sanders pushed back on campaign staffers' pay complaints and voiced frustration that some went to the media

Democratic presidential candidate Elizabeth Warren gestures as she speaks during a campaign stop at George Mason University in Fairfax, Virginia on May 16, 2019. MANDEL NGAN/AFP/Getty Images

There are some key distinctions in Sanders and Warren's political philosophies and among their supporters

On the surface, Warren and Sanders may seem like close to carbon copies of each other.

After all, they're both progressive New England senators running on clear, strong economic populist messages that emphasize redistributing wealth, reducing the power of large corporations and the ultra-wealthy, and placing more economic and political power in the hands of the working class.

But there are some crucial differences in how Sanders and Warren view the Democratic primary and their roles in it.

As Aidan Smith argued in a recent piece for The Nation, Sanders views himself as the leader of an anti-establishment movement that serves as an adversary and challenge to the Democratic party apparatus, whereas Warren is more willing to work within it.

"Sanders sees the party as an obstacle to meaningful progress, a box within which he is forced to work given the catatonic nature of the American two-party system," Smith wrote, adding, "In Warren's view, the Democratic Party is, if not good, an acceptable vehicle for political change in need only of minor tweaking, rather than a complete makeover."

Not only do they have different philosophies of where they fit in the Democratic party, they also hold subtle but important distinctions in their political philosophies.

While Sanders has labeled himself a Democratic socialist, Warren hasn't ascribed that label to herself and identifies herself as a capitalist, telling CNBC in 2018, "I believe in markets. What I don't believe in is theft, what I don't believe in is cheating."

Summarizing a separate piece by Shawn Gude in the socialist publication Jacobin, Smith wrote that according to another theory of their differences, "Sanders' immediate priorities are direct redistributionist policies, while Warren, reputed as a policy wonk, is most interested in systemic changes to the American market economy."

In the debate, Sanders may directly or subtly imply, as he did in his June 19 tweet, that Warren's comparatively friendlier stance to the Democratic establishment means she can't truly be trusted to carry out her stated mission of taking on big, structural changes to the American economy and political society.

While most voters aren't yet fully decided, the two share some overlapping support. According to INSIDER's recurring SurveyMonkey Audience poll, 48% of Sanders' supporters would be satisfied with Warren as the nominee, while 61% of Warren supporters would be satisfied with Sanders.

Read more: How the INSIDER 2020 Democratic primary tracker works

But as Politico reported earlier this month, a closer look at the currently available Democratic primary polling indicates that the two candidates' current bases of support aren't completely interchangeable.

While Sanders outperforms Warren among young, male, non-college educated, and African-American voters, Warren leads with highly-educated, older, and higher-earning voters in Morning Consult's polling, as well as with voters who described them as "extremely interested" in politics.

Former Sanders strategist Mark Longabaugh made a similar argument about the two candidates' different appeals to Smith, telling Politico, "Warren is identifiably a Democrat and runs as a Democrat ... whereas Bernie very intentionally ran for reelection as an independent ... and appeals to those who look inside the Democratic Party and think it's not their thing."