With Sen. Bernie Sanders gainining momentum in the race to win the Democratic nomination for president, some Democrats fear his unprecedented candidacy could end up hurting House Democrats.

As they see it, an avowed Democratic socialist like Sanders and his promise of a political revolution including sweeping policy proposals is completely divisive and unpalatable to scores of voters.

But the evidence is more complicated. In all, Republicans don't have a great shot at winning back the House majority this year mainly because of their massive fundraising disadvantage.

Sanders – and down-ballot Democrats — are also set to benefit from the ongoing trends of negative polarization and asymetric polarization in American elections, and it's not clear that he would be a weak nominee.

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Democrats pulled off a huge upset in the House in the 2018 midterm elections, flipping 40 previously GOP-held House seats and allowing Democrats to both pursue important legislative initiatives and impeach Trump.

But as Sen. Bernie Sanders gains momentum in the race to win the Democratic nomination for president, some Democrats fear his unprecedented candidacy could end up hurting Senate, House, and state legislative candidates all the way down the ballot.

The New York Times reported on Thursday that since Sanders' blowout Nevada caucus win has caused an all-out crisis among the Democratic establishment. According to the Times, who interviewed 93 of the senior Democrats and DNC members who serve as "superdelegates," these insiders are "not just worried about Sanders's candidacy, but are also willing to risk intraparty damage to stop his nomination at the national convention if they get the chance."

As they see it, an avowed Democratic socialist like Sanders and his promise of a political revolution including sweeping policy proposals like single-payer health insurance is divisive and unpalatable to scores of voters who have unfavorable views of Democratic socialism, with Democratic House and Senate leaders hearing "constant warnings from allies about congressional losses in November," The Times said.

At the February 25 Democratic debate in South Carolina, Sanders' opponent former Mayor Pete Buttigieg went as far as to charge, without evidence, that Sanders at the top of the ticket would result in Democrats losing their control of the House majority altogether.

And this week, Sanders' controversial comments praising some elements of Cuban dictator Fidel Castro's regime set off particular fury from many moderate House Democrats in Florida, a crucial swing state with a large population of Cuban refugees who fled Castro's Cuba and their descendants.

The concerns of some vulnerable House Democrats aren't unfounded. In 2018, after all, voters strongly expressed their dislike of the incumbent president Trump by voting out their Republican representatives in favor of Democrats.

And no vulnerable House Democrats have endorsed Sanders — those that have endorsed are backing more moderate candidates including former VP Joe Biden and former Mayors Mike Bloomberg and Buttigieg.

"South Carolinians don't want socialism," Rep. Joe Cunningham, a Democrat who flipped a Republican seat in South Carolina, recently told the Post & Courier ahead of the state's Saturday primary. "Bernie's proposals to raise taxes on almost everyone is not something the Lowcountry wants and not something I'd ever support."

To put it simply, it's far too early to decisively declare whether or not a Sanders candidacy would be a killer for down-ballot Democrats and cost them control of entire legislative chambers.

While Sanders seems in the strongest position at the moment, only three states accounting for about 3% of delegates have voted in the Democratic nomination process so far, meaning his nomination isn't close to being guarateed yet.

Even assuming Sanders is the nominee, there simply isn't enough evidence yet to determine the exact down-ballot impacts of his nomination. But we do have evidence about broad trends in American elections today that give us an idea of how it could play out.

Rep. Elissa Slotkin, D-Mich. Associated Press

How the race for control of the House currently looks

Before delving into the specifics of how a Sanders nomination would affect down-ballot races, here's how the House currently stands.

All in all, Republicans are highly unlikely to take back control of the House in 2020, but there are definitely lots of highly competitive battleground seats located in red or swing states that they could have a good shot of flipping back with presidential-year turnout.

Out of the 40 congressional districts Democrats flipped in 2018, 21 were won by Trump in 2016, while 19 were carried by Hillary Clinton. Additionally, ten House Democratic incumbents won re-election in 2018 representing seats Trump carried.

There are currently 233 Democrats, 197 Republicans, and five vacant seats in the House of Representatives. Three of the vacancies are from Republican-held districts that are likely to remain in GOP hands after special elections, and two are from Democratic seats that are also likely to remain Democratic.

Assuming that all five vacancies are filled by November and stay within the same party, Republicans would need to win back 19 seats in order to flip the House completely and get to a 218 seat majority.

While there are plenty of nuances, exceptions, and caveats to these categories, the seats Democrats flipped in 2018 largely fall into two main buckets:

Suburban districts, many located in blue or purple states with wealthier, college-educated residents, that were trending towards Democrats for several cycles and probably won't return to Republican control anytime soon. New Jersey's eleventh district in the New York suburbs, Virginia's 10th district outside of DC, the 26th and 27th districts in South Florida, Minnesota's second and third districts in the Minneapolis suburbs, Colorado's 6th district in the Denver suburbs, Illinois' 6th district in the Chicago suburbs, California's 49th district and several others in Orange County, and Arizona's 2nd district all fall into this category.

More rural or ex-urban congressional seats, many located in red or purple states, that voted for Trump in 2016 in some cases by large margins and aren't trending Democratic at the same rate, if it all.

Here are just some of the most vulnerable freshman House Democrats, all of whom represent seats that largely fall into the second category that Trump won by margins of five percentage points or more in 2016, according to an analysis in The Hill:

Rep. Anthony Brindisi of New York's 22nd district, which Trump won by 15.5 percentage points in 2016

Rep. Kendra Horn of Oklahoma's fifth district, which Trump won by 13.4 percentage points

Rep. Joe Cunningham of South Carolina's first district, which Trump won by 13 percentage points

Rep. Jared Golden of Maine's second district, which Trump carried by 10 percentage points

Rep. Xochitl Torres-Small of New Mexico's second district, which Trump won by 10 percentage points

Rep Max Rose of New York's 11th district, which Trump won by 10 percentage points

Rep. Elissa Slotkin of Michigan's eighth district, which Trump won by seven percentage points

Rep. Antonio Delgado of New York's 19th district, which Trump won by seven percentage points

Rep. Ben McAdams of Utah's fourth district, which Trump won by nearly seven percentage points

Rep. Abigail Spanberger of Virginia's seventh district, which Trump won by six percentage points

Rep. Andy Kim of New Jersey's third district, which Trump won by six percentage points

This means that in order to flip the House, Republicans would need to win back not only the six Democratic-held districts that Trump won by double-digit margins, but also several districts he won by smaller margins that are slipping out of Republican hands due to broader demographic realignments of suburban voters moving away from the GOP.

Democratic congressman Antonio Delgado. Seth Wenig, File via AP

Two swing districts in Michigan and New York are possible case studies

Some districts, like Slotkin's, that went for both Mitt Romney in 2012 and Trump in 2016, are located in the type of suburban areas that are trending Democratic throughout the nation, and in her case, is in a battleground electoral college state.

"There is that character out here where those people might be turned off by Sanders," Corwin Smidt, an associate professor of political science at Michigan State University in nearby Lansing, told Insider.

Smidt said most concerns about a particular nominee spoiling down-ballot races are "generally overrated," but certain pockets of affluent, college-educated white voters in suburban Michigan could tip the scales in a district like Slotkin's.

Over the midterms in 2018, Smidt pointed to the poor performance in these precincts by Sanders-backed gubernatorial challenger Abdul El-Sayed, who lost to the eventual governor, Gretchen Whitmer, by 22 percentage points in the Democratic primary.

In Delgado's district in upstate New York, a much more rural area which voted for Barack Obama in 2012 and swung to Trump in 2016, local observers say Delgado is well-positioned for 2020 but Sanders could pose problems in other similar districts.

Former GOP congressman John Faso, who lost to Delgado in 2018, told Insider: "I haven't fully assessed [Bernie's candidacy] but I do think that his views are so extreme and out of the mainstream...that his candidacy is gonna be a real drag on Democrats in swing areas across the state."

Brindisi, who represents an upstate New York seat in the Syracuse area that Trump won by over 15 points, told Syracuse.com he does not support Warren or Sanders and thinks it would be "exceedingly difficult" for either to win in his district, citing their plans to ban most private health insurance as a particularly serious liability.

Professor Gerald Benjamin, the director of the Benjamin Center for Public Policy Initiatives at SUNY New Paltz, told Insider he thinks Delgado at least is "a very strong candidate and I think that he's going to win on the merit of his own performance," but that Sanders could pose a bigger problem for representatives in other places.

But aside from the long-term demographic and partisan realignments affecting both parties, Republicans would have a lot of catching up in the money race to do to try to win back the chamber in this particular cycle.

As Politico recently reported, House Democrats and the DCCC currently have a huge fundraising advantage over their Republican counterparts, with the approximately 60 most vulnerable House Democrats raising "a collective $28.5 million in the last three months of 2019, a staggering total that is nearly twice the sum of all of their Republican challengers combined."

In one-closed door meeting in late January, House Minority Leader Rep. Kevin McCarthy reportedly described the Democrats as "kicking our ass" when it comes to fundraising, with NRCC Chair Rep. Tom Emmer warning that "the campaigns that need to get better with their individual fundraising" are "crushing us."

The cases for and against Sanders tanking Democrats' control of the House

There are two main theories as to how a Sanders candidacy could hurt Democrats down the ballot. While both certainly have merit and could end up coming to fruition in November, the actual evidence is more complicated.

The first theory, advanced by people including the leaders of centrist Democratic group Third Way, rests on the assumption that Sanders would be a weak candidate against Trump, and because of the rise in negative polarization and the decline in ticket-splitting at the federal level, Sanders would lose the presidency and bring several House and Senate seats down with him.

In a recent piece for The Atlantic, conservative staff writer David Frum wrote that "upmarket districts voted to reprimand the president's language and behavior. A Sanders nomination invites those districts to vote in 2020 to raise their taxes and replace their health insurance. That may be a tougher sell."

Frum's equivalency between a Trump and Sanders presidency, however, is somewhat of a misleading one. While Trump has complete control over his own "language and behavior," Sanders cannot unilaterally raise taxes or abolish private healthcare with the stroke of a pen.

Indeed, as Sanders' own surrogates have acknowledged, the vast majority of Sanders' most sweeping and ambitious policy proposals have essentially a 0% chance of passing through a Republican-controlled or even a Democratic-controlled Senate, which requires a 66-seat, filibuster-proof supermajority to pass any programs with appropriations, or government spending.

As Vox's Matthew Yglesias recently argued, "On the vast majority of issues, a Sanders administration would deliver pretty much the same policy outcomes as any other Democrat. The two biggest exceptions to this, foreign policy and monetary policy, happen to be where Sanders takes issue with an entrenched conventional wisdom."

Just as all House Democratic candidates were closely linked to Pelosi, Republicans will be sure to tie the eventual Democratic nominee to socialism, especially in the form of Medicare for All and the Green New Deal.

Some may argue that Sanders, who has proposed a moratorium of most deportations, decriminalizing illegal entry down from a misdemeanor to a civil citation, and extending Medicare for All benefits to undocumented immigrants, would be particularly vulnerable to efffective attacks on his immigration policies.

Throughout the 2018 midterms cycle, the Republicans tried a similar message to defend their then-House majority.

The Republicans villainized Speaker Nancy Pelosi as a coastal elite socialist, attacking Democratic candidates as Pelosi-lite versions of her, and hammered hard on immigration, stoking fears of a migrant caravan approaching the US and highlighting gruesome cases of crimes committed by undocumented immigrants.

In the end, House Democratic candidates didn't take the bait, and mainly focused on tying Trump to so-called kitchen table issues like healthcare, prescription drug costs, and taxes (which are also the main issues Sanders centers in his campaign message), resulting in House Republicans losing 40 seats in a blowout defeat.

As many analysts including the conservative writer David French of The Dispatch have argued, there's plenty of evidence that Sanders would be a formidable general election opponent against Trump.

The evidence on whether Sanders would turn out enough young voters and non-voters into his coalition to offset his expected weaknesses among older and possibly more moderate voters is somewhat mixed so far, with political scientists Jacob Kalla and David Broockman arguing that Sanders leading Trump in most preliminary head-to-head match-up polls "relies on some remarkable assumptions about youth turnout that past elections suggest are questionable."

French points out that the rise of negative polarization, or the phenomena voters being driven by strong dislike of the other party instead of loyalty to a party, will advantage Sanders in a contest against Trump — and the party he now controls — no matter what.

As Vox founder Ezra Klein explains in his recent book "Why We're Polarized," negative polarization, which empirical studies show has been on the rise over the past century in the form of lower ticket-splitting down-ballot, refers to "partisan behavior driven not by positive feelings toward the party you support but negative feelings toward the party you oppose."

Klein points out that negative polarization doesn't just apply to self-described partisans but also to independents and leaners, writing that "a 2016 Pew poll found that self-described independents who tended to vote for one party or the other were driven more by negative motivations" than positive ones.

After all, Sanders isn't running against a generic Republican but against Trump, a uniquely unpopular president and flawed candidate for re-election, setting Sanders up to be even more advantaged by the forces of negative polarization.

As Kyle Kondik, managing editor at the University of Virginia's Center for Politics' Crystal Ball, put it in a recent op-ed for The New York Times: "Mr. Sanders being a potentially weak opponent doesn't necessarily make the president a beloved incumbent."

Reports have suggested Russia was interested in providing support to the Sanders and Trump campaigns. Mike Segar, Alexander Drago/REUTERS

Could a Sanders candidacy hurt down-ballot Democrats even if he wins?

The other theory of how a Sanders candidacy could hurt Democrats, advanced in part by Frum, argues that even if Sanders wins the presidency, enough voters in competitive districts will anticipate a Sanders victory and strategically vote for Republican representatives as a check on his administration, costing Democrats crucial House and Senate seats.

As Kondik noted, research from Columbia University elections scholar Robert Erikson shows that this type of ticket-splitting is prevalent in American presidential elections.

This type of strategic voting actually relies on voters believing that Sanders will win — and Kondik pointed to the recent 2016 election as an example of a polarizing and highly unpopular nominee both winning the presidential election and avoiding down-ballot bleeding.

In 2016, for example, Republican leaders and elites similarly worried that Trump at the top of the ticket would result in massive backlash and losses at both the House and Senate levels.

But many districts voted to re-elect their Republican representatives under the assumption that Clinton would win, ultimately resulting in a net loss of just six House seats for Republicans running on the same ticket as an incredibly unpopular nominee who underperformed most GOP House members.

On the Senate side too, 2016 had the lowest levels of ticket-splitting in modern history, with the Republican candidate winning in every state with a concurrent Senate race that Trump also carried.

In order for voters to strategically vote for Republicans as Erikson's research and Kondik's piece outline, they have to believe that Sanders would be favored to win in the first place, which currently isn't the case.

While these figures could drastically change over the next several months, two-thirds of Americans said they predicted that Trump will be re-elected in the fall according to a recent Monmouth University poll, an effect that Kondik said is likely to be compounded if mainstream Democratic politicians further drive pessimistic media coverage about Sanders' chances against Trump and possibly driving voters to elect down-ballot Democrats.

"It's far too early to say for sure, but I'm skeptical Sanders would drag down Democrats in most suburban places Democrats won in 2018," Dave Wasserman, the U.S. House races editor at the Cook Political Report, told Insider. "And the reason is voters in those districts tend to have college degrees, and cast their ballots strategically."

It's undeniable that Sanders' candidacy could pose a big threat to freshman Democratic House members in red states who came into office in the 2018 blue wave but who have to run for re-election in a presidential year in red or purple-state districts likely to go for Trump again.

But due in part to negative polarization, red-state Democrats like Rep. Kendra Horn in Oklahoma and Rep. Joe Cunningham in South Carolina will likely face serious threats to their re-election bids regardless of which Democrat is at the top of the ticket.

And one sign of hope for those members lies in the asymmetric, or uneven, polarization between the two parties, which in practice, leads to fundamental differences in how they organize themselves and achieve their goals.

As political scientists Matt Grossman and David Hopkins explain in outlining their theory of asymmetric polarization, the Democratic party operates more like a broad, multifaceted coalition of members representing different policy interests as opposed to the Republican party, which especially under Trump functions more as the "vehicle of a conservative ideological movement."

Despite the rise in negative polarization and the resulting decline in ticket-splitting, this dynamic of Democrats being more focused on policy outcomes than ideology makes it much easier for Democrats to express dissent and actively distance themselves from the figurehead of their party than it is for Republicans.

In 2018, as a group of moderate House Democrats including Reps. Seth Moulton, Tim Ryan, and Kathleen Rice launched an effort to block Pelosi from becoming Speaker, Pelosi implicitly gave several newly-elected vulnerable House Democrats, including Slotkin, Cunningham, and others leeway to openly criticize her and vote against her bid for Speaker.

Those sorts of defections simply aren't feasible on the Republican side. As Democrats have won back the House, Wasserman noted that on the other side of the aisle, a full 47% of the Republican representatives who were in the chamber when Trump took office have either lost re-election, retired, or are retiring in 2020.

Looking at the broader picture in places like Michigan and beyond, Democrats have to think about the long-term demographic and party re-alignment shifts at play over the next several cycles — not to mention the upcoming redistricting after the 2020 census.

As the Michigan State professor Smidt wondered of House Democrats, "Are they branding themselves for now or for the future?"

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