Editor's Note: John Avlon is a CNN senior political analyst. The opinions expressed in this commentary are his own. View more opinion articles on CNN.
(CNN) - Democrats should be paying close attention to the Labour Party's brutal beating in the UK elections last night.
It is a cautionary tale about the perils of polarization and the predictable dangers of embracing a far-left leader.
On the one side, the UK had an unpopular conservative incumbent; a charismatic nationalist with a persistent truth-telling problem who demonized dissent within his own party.
After Conservatives lost seats under Theresa May amid a tortured season of Brexit deals that couldn't get through Parliament, Labour should have been primed to take back 10 Downing Street.
But they decided to stick with a far-left populist and unreconstructed socialist, Jeremy Corbyn, someone with a muddled message on Brexit, who is dogged by accusations of failing to root out anti-Semitism in the Labour Party, calls Karl Marx a "great economist" and has a record of admiration for leftist autocrats from Cuba to Venezuela. He promised to nationalize rail and energy industries and spend hundreds of billions remaking society. (Corbyn has stood by his efforts to address anti-Semitism in his party.)
To put it in an American context, Jeremy Corbyn makes Bernie Sanders look like Bill Clinton.
But he was beloved by the party's base and young voters who boosted him on social media.
And so despite polls showing that Corbyn was the least popular opposition leader in almost a half century -- with a stunning 76% of all Britons saying that they were unhappy with him, they decided to take the leap, convinced that the unpopularity of Boris Johnson, chaos over Brexit and support for the Labour platform meant that maybe they could sneak Corbyn into 10 Downing Street.
Not so much. Actually, not at all.
Under Corbyn, the Labour Party had its worst showing since 1935. It lost seats in former Labour strongholds throughout the middle of the country. They lost working class voters to the Tories. It was a fierce repudiation of Corbyn's radical plan to remake Britain.
Even in a parliamentary system, elections are a compared-to-what proposition, and Boris Johnson -- a former Mayor of London and Brexit cheerleader who campaigned as a self-described "modern progressive conservative"-- cleaned up despite a slight decline in turnout.
This should be a time for reassessment in Labour. Its defeat was both predictable and avoidable. While many on the left now dislike Tony Blair with a passion only second to Margaret Thatcher, the stark fact remains that Blair's centrist New Labour won three consecutive elections -- something no other Labour leader has done -- while more left-wing candidates before and after him have lost badly.
There are no perfect political parallels between different countries, but in the UK as in the USA, we see how polarization distorts democracies and elevates the most extreme voices - depriving voters of representative policies and often a functioning government.
The record of the past half-century makes a compelling case that the center holds when it's given a strong voice...but in its absence, right-wing populism beats left-wing populism.