After New Hampshire, the Democratic Party's historic establishment is worried Sen. Bernie Sanders could win the 2020 presidential nomination.

They fear Sanders will take the party to the left, and get crushed in November by President Donald Trump.

There are a lot of parallels between what is happening to the Democratic Party right now and what happened to the UK's Labour party under Jeremy Corbyn.

Sanders may be the candidate hardcore Democrats want — but you have to persuade voters from the other party to join you, to win a presidential election.

Trump and his supporters are hoping Sanders will win, too. They think Sanders would be easy to beat.

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Former Vice President Joe Biden's poor showing in the New Hampshire and Iowa primaries this week — and Bernie Sanders' strong performance in both — are terrifying the Democratic party's top managers and fundraisers.

We got to taste that fear when Obama-era Secretary of State John Kerry was overheard on a phone call in a hotel lobby in Des Moines, Iowa, telling someone that voters "now have the reality of Bernie" and it would lead to "the possibility of Bernie Sanders taking down the Democratic Party — down whole."

In a viral rant on MSNBC, James Carville — the Democratic strategist who famously got Bill Clinton into the White House — also said he was "scared to death" of what he was seeing.

Tellingly, he compared Sanders to Jeremy Corbyn, the left-wing leader of Britain's Labour Party, who in December lost an election to the Conservative Party's Boris Johnson.

Top Democratic Party strategist James Carville told MSNBC on Tuesday he is "scared to death." MSNBC

Carville and Kerry's fear is that the enthusiasm of their party's activists and hardcore voters for a left-wing candidate — namely, Sanders — to face President Donald Trump may eclipse a central fact: In order to win an election, you have to persuade moderate voters from the opposite party — not your own — to support you.

It is difficult to see how Sanders, a democratic socialist, is going to persuade the moderate conservatives he will need to beat Trump.

Parallels between Bernie Sanders and Jeremy Corbyn

The driving force behind these top Democrats' fear is what they have just seen happen to the Labour party in Britain. In 2015, Labour elected Jeremy Corbyn as its leader, amid an exciting wave of left-wing activism and energy.

But voters outside the party didn't buy it. Under Corbyn, Labour lost two national elections in three years (with Brexit dividing voters even more). The party is now a shell of its former self, commanding less than a third of the seats in the UK parliament. Corbyn has been a disaster for Labour. He took the party "down whole."

There are a lot of parallels between Corbyn and Sanders:

Both are democratic socialists.

Both are men of principle, who spent years in the political wilderness before vaulting to their party's top echelons.

Both are old — in their seventies.

Both have been embraced as agents of radical change by young activists who are fed up with the crushing inequalities of post-internet capitalism.

Both are proven vote-getters inside their own parties.

Jeremy Corbyn. Reuters / Samantha Lee, Business Insider

"The Sanders people are taking pictures wishing Jeremy Corbyn the best ... I don't want to go down that path"

But Carville and Kerry know you need more than your own party's votes to win.

Carville said on Tuesday night: "It matters who the candidate is, it matters what the party chooses to talk about ... I am scared to death."

"All the Sanders people are taking pictures wishing Jeremy Corbyn the best," he added. "I don't want to go down that path."

Their concern is legitimate, if you believe there are other parallels between the 2019 general election in Britain and the 2020 presidential election in the US:

The incumbent Conservative/Republican candidates, Trump and Johnson, are both populists.

They both have cartoonish ways of behaving and talking.

They both have a long history of telling outrageous lies, and voters not caring.

They both have a long history of making racist and sexist statements, and voters not caring.

They are both enjoying robust economies with low unemployment.

And their opponents on the left are assuming that both of them are so obnoxious they must lose at the ballot box.

Sanders is the candidate Trump wants to win. REUTERS/Brendan McDermid

Working-class voters have shifted to the right

For the first time ever, in 2019, a majority of working-class people in Britain voted Conservative rather than Labour. That shift of working-class votes to the right should sound familiar to Americans — it's how Trump won the white, working-class, rust belt states from Hillary Clinton in 2016.

The working-class vote will be key to a Trump victory, just as it was key to Johnson's.

I recently asked the former GOP election strategist Matthew Gagnon, who helped me predict that Trump would win in 2016 when no one thought he had a chance, what he thought would happen in November.

"Trump will be very strong, and if he does lose, it will be a reasonably close election," he said.

"But he will remain strong in the rust belt, Pennsylvania, and the Great Lakes region. He may lose some or all of those states, but he'll be strong there and compete mightily."

"If you asked me to predict today, I would probably tell you that I think he gets re-elected, but with a slightly smaller electoral vote margin than he had last time."

The fate of the left when times are good

Although the Corbyn defeat was partly unique to Corbyn himself — his personal politics were toxic to moderate and conservative voters, a YouGov analysis found — it was greater than him, too.

British voters have now been offered four different manifestos of democratic socialism, under three different leaders from the right, left and middle of the Labour party, and been rejected every time since 2010. It's an astonishing record of failure for an organization that was the UK's natural party of government between 1997 and 2010.

There are myriad reasons for those defeats.

But Carville's TV appearance reminded me of the most obvious one, which is another parallel with the US: since 2010, both countries have enjoyed continuous economic growth with ever-lower unemployment.

'It's the economy, stupid'

In 1992, when Carville was plotting Clinton's ascent to the White House, he coined the phrase "It's the economy, stupid."

The phrase asserts that regardless of what's going on in the media, voters are largely driven by one overriding factor: whether they feel better off today than they did before.

Bill Clinton won because the US was in recession under Republican President George H.W. Bush, and voters wanted a change. The US today, under Trump, is booming.

If it really is "the economy, stupid," then Democrats face an uphill battle to persuade voters who have done well under Trump to switch parties.

Biden polls better than Bernie

According to polls tracked by Real Clear Politics, Biden consistently polls more strongly against Trump than Sanders does. Here is the most recent one from NBC/Wall Street Journal.

The GOP is so aware of this fact that it is encouraging its own members to re-register as Democrats in order to vote for Sanders in the primaries.

They want Sanders to win because they see him as the easiest enemy to defeat.

'Do we want to be an ideological cult?'

That's why Kerry and Carville are so scared by what just happened in Iowa.

"We've got to decide what we want to be," Carville told MSNBC. "Do we want to be an ideological cult? Or do we want to have a majoritarian instinct to be a majority party?"

"What we need is power, you understand? That's what this is about," he said. "Without power, you have nothing. You just have talking points."