Bernie Sanders is too extreme to win the White House, the Hillary Clinton–backing Super PAC Correct the Record suggested in an opposition research memo.

The group sent an email to the Huffington Post that compares Sanders to socialist British Labour Party leader Jeremy Corbyn and the late communist- and socialist-backed Venezuelan President Hugo Chavez, Samantha Lachman and Ryan Grim reported Monday night.

It's neither new nor surprising that Correct the Record is trafficking in opposition research — its purpose is to help Clinton win — but the email comes at a time when Clinton herself is making clear that she's ready to battle Sanders head-on.

As I wrote Monday morning, Clinton's shift to offense includes defining herself as a "moderate" against the liberal Sanders, who has described himself as a socialist, and talking about how she will begin to contrast her platform with his when Democrats hold their first primary debate next month.

Sanders has said he won't run negative ads against Clinton, but he is using the tried-and-true method of pointing out their differences on policies that animate the Democratic base. And it appears to be part of what's working for Sanders, as Clinton's poll numbers have declined precipitously over the summer. Sanders holds leads in Iowa and New Hampshire, the states with the first two primary contests next year, according to the most recent polls.

It all comes down to this: There's a real fight for the Democratic nomination — not the coronation of Clinton that many of the party faithful feared — and it is going to be waged in part on the questions of whether Sanders is too liberal and whether Clinton is too moderate.

Why the Labour Party leader is relevant to the Democratic primary

To understand what Correct the Record is trying to do, you first have to know a little bit about recent British politics and the Labour Party's recent turn to the left. Once led by the more centrist Tony Blair, the party picked Corbyn, a socialist, as its leader this past weekend.

Vox's Matt Yglesias wrote this week that it was a reaction to the Conservative Party's move to the center, which includes support for LGBTQ rights, calls for action on climate change, and limiting the depth of tax cuts. Rather than respond by fighting for the middle ground, Labour pushed further left by electing Corbyn.

Matt was elaborating on a tweet from the New York Times's Neil Irwin, who observed that the Democratic Party in the US would face a similar choice if Republicans dashed to the center, and that it might not be a pretty fight for Democrats.

A Corbyn theory: If the US Republican party was as centrist as the UK Tories, Democrats would face the same existential dilemma Labour does. — Neil Irwin (@Neil_Irwin) September 12, 2015

Sanders cheered Corbyn's victory.

"At a time of mass income and wealth inequality throughout the world, I am delighted to see that the British Labour Party has elected Jeremy Corbyn as its new leader," he said.

Correct the Record is saying Sanders is too far left to win the presidency

The implication of the Correct the Record attack on Sanders is that Sanders could drive the Democrats away from a winning formula and off the left edge. It notes Corbyn's depiction of members of the terrorist groups Hezbollah and Hamas as "friends," his lament that there was no attempt to capture rather than kill Osama bin Laden, and his frustration at NATO's expansion into Eastern Europe.

Here's the thrust of what Correct the Record said in comparing Sanders to Corbyn, per HuffPo:

The "similarities" between the two, according to the email, include Sanders' introduction of legislation to terminate the United States' nuclear weapons program, comments that NATO's expansion into former Soviet states is dangerous because it could provoke Russia, opposition to more U.S. funds for NATO, and saying he "was concerned" that proposed new NATO members had shipped arms to Iran and North Korea.

It also notes Sanders's role in supporting a deal with Chavez that provided heating oil to low-income people in New England nearly a decade ago — a program championed by former Rep. Joseph Kennedy II.