A Bernie Sanders campaign rally in Chicago, Illinois. Scott Olson/Getty Images

In a new INSIDER poll, around 30% of Democratic respondents said the party could lose the 2020 presidential race because the Democratic nominee "failed to recruit Republican supporters."

It was closely followed by 29% who said they feared the Democratic nominee would fail "to turn out the base."

That means a nearly equal amount of self-identified Democratic primary voters are worried about the party becoming too moderate or too progressive in 2020.

The results reflect the ideological gulf among Democrats over the most effective political approach against President Donald Trump.

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Nearly an equal amount of self-identified Democratic primary voters responded in an INSIDER poll that their party could blow it in the 2020 presidential election if they weren't both progressive or moderate enough. The results reflect the ideological gulf among Democrats over the most effective political approach against President Donald Trump.

The INSIDER poll asked respondents, "If the Democratic nominee were to lose to Donald Trump in the presidential election, why do you think that would be?"

Around 30% Democratic respondents said they could lose because the party's nominee "failed to recruit Republican supporters." It was closely followed by 29% who said their nominee "failed to turn out the base."

The question of whether the Democratic Party should adopt centrist positions to win back the white-working class voters who fled the party for Trump, or to embrace a progressive agenda and rely on a coalition anchored by voters of color and young people, have thrown the party into an existential debate over its future.

Read more: The 2020 presidential election could come down to college students and that could be trouble for Trump

The primary has been characterized by progressive and moderate candidates charting diverging paths towards healthcare reform, combating climate change, and alleviating economic inequality.

Candidates like Sens. Bernie Sanders and Elizabeth Warren have successfully lurched Democrats to the left with their full-throated support of "Medicare for All," the Green New Deal, and substantially reducing student debt.

But they've faced criticism from moderates like former Vice President Joe Biden, who argues Democrats need to chart a centrist course to defeat Trump. He's pointed to the 2018 midterms where moderates won enough swing districts to restore the party's control of the House.

At the July Democratic primary debate recently, former Colorado governor John Hickenlooper, another moderate, laced into Sanders for embracing "Medicare for All," and criticized his progressive policies as "a disaster at the ballot box."

Read more: Here are all the 2020 Democratic presidential candidates who have qualified for the September primary debates

"You might as well FedEx the election to Donald Trump," Hickenlooper said, echoing the concerns of moderate voters who believe the party's sharp leftward turn could cost it dearly in next year's presidential election against Trump.

For the poll, INSIDER asked over 1100 respondents to identify why a Democratic nominee could lose to Trump and identify one or more reasons for it. Then the respondents were provided with 16 options with the ability to choose multiple answers. 492 self-identified Democrats answered.

The two highest-ranked causes for a defeat were rooted in the party's identity, highlighting the ongoing Democratic debate on whether it should seize on its progressive roots or play it safe with a centrist platform that could win over more independents and Republicans disaffected with the Trump presidency.

Among the concerns Democratic voters have about potentially losing the 2020 presidential election, INSIDER also found in its polling:

Nearly 42% of Democratic respondents feared party voters would fail to turn out.

Around 32% said the Democratic nominee would fail to expand the map into states that previously backed Republicans but are more favorable to them next year.

28% said the Democratic nominee wouldn't win traditionally Democratic states that went for Trump — which happened in 2016.

26% of respondents worry the Democratic nominee will be too far to the left.

13% said Democrats are concerned their party's candidate won't be as well-funded as the president's campaign, signaling a lack of enthusiasm for the eventual nominee.

12% said they worry the Democratic nominee will be too centrist to motivate liberal supporters.

These results won't provide comfort to Democrats looking for the most effective strategy to beating Trump in 2020, instead showing an electorate divided on the way forward.

FiveThirtyEight's Nate Silver pointed out on Twitter recently there is a slew of political science research supporting the argument that moderates are more likely to win general elections. The conventional wisdom is that candidates move toward the end of the spectrum appealing to ideologues in the primary, only to revert to the center during the general election to win over moderates.

In 2016, though, Trump ran on a ultraconservative stance on immigration both in the primary and the general election against Hillary Clinton, preferring to energize the Republican base and suffering little electoral consequences for not moderating his stance. He lost the popular vote, but won the presidency by a decisive margin through the electoral college.

Read more: There are 2 dozen 2020 Democratic presidential candidates, but it's really only a 5 person race

However, recent data shows moderates may be losing their electoral edge.

Boise State University political science professor Dr. Stephen Utych researched whether moderates or progressives had the upper hand in general elections. He found that in 1980, moderates won far more elections, but almost tied with ideologues in 2008 and lost more often than not in 2012.

Though Democrats are united in their desire to beat Trump, they largely disagree on the prescription for its future. They face an election in 2020 where they have to turn out Democratic voters who stayed home in 2016 and not dampen the energy of its progressive base. How the party strikes the right balance between both remains an open question.

SurveyMonkey Audience polls from a national sample balanced by census data of age and gender. Respondents are incentivized to complete surveys through charitable contributions. Generally speaking, digital polling tends to skew toward people with access to the internet. Total 1184 respondents collected from July 19-20, 2019, a margin of error plus or minus 3.01 percentage points with a 95% confidence level.