The destruction of the centrist Democrats in the U.S., Blue Dog Democrats in particular, is well known. What isn't talked about as much is how the center-left is being routed all over the western world.

The spectre that haunts Europe’s centre left has a name: Pasokification. In 2009, Greece’s once-great social democratic party won 43.9% of the national vote. Barely six years later, it could manage just 6.3%.
Atomised in France, all but wiped out in the Netherlands, humiliated in Germany, Europe’s mainstream centre left is in full retreat. Even in its one-time stronghold of Scandinavia, social democracy is now struggling.
There are many reasons. The embrace-the-market “Third Way” policies of leaders such as Tony Blair and Gerhard Schröder worked fine in the turn-of-the-century boom years but seem to offer little to today’s vulnerable centre-left voters.

Germany's Gerhard Schröder is a good example. His cluelessness can only be matched by Hillary's cluelessness.

There are few better illustrations of this crisis than Social Democrat leader Martin Schulz's futile anger at Chancellor Angela Merkel in the aftermath of Sunday's election. Schulz called her "the biggest loser" of the race; he chastised her for "a systematic evasion of politics" that created a vacuum for nationalists to step in and declared her feel-good campaign "scandalous." He made it sound as though it was Merkel, whose Christian Democratic Union governed with his SPD for the last four years, who caused his party to show its worst ever election result -- just 20.5 percent of the vote. Considering that he'd campaigned against her, and that she's won the election, Schulz's charges sounded surreal. Clearly, he has no clue what happened to his party's support, just as French Socialists had no idea why their backing collapsed in the run-up to this year's presidential election, and just as the Dutch Labour Party struggled to comprehend its electoral implosion in March.

The center-left (i.e. "socialist" parties in Europe) had been getting killed since 2009, but 2017 was a particularly bad year.
It appears that 2018 will be more of the same.

Ask anyone in Italy about who might be sitting at the head of government in Palazzo Chigi following national elections on 4 March and the answer is a collective shrug of the shoulder.
...There is only one likelihood that most pundits do agree upon: that the Italian centre-left, led by former prime minister Matteo Renzi, seems poised to be heading for a humiliating defeat, not unlike its ideological counterparts in Germany and Spain.
...“The centre-left is not seen as an alternative to the disgruntled voters out there. It embraced globalisation and bailed out banks when needed, and has not been able to differentiate itself from centre-right,” says Wolfango Piccoli, the co-president of Teneo Intelligence, a research firm in London.

This is not to say that we have a Crisis Of The Left.
When center-left parties turn to the left the public responds.

A good example is the UK Labour Party under socialist Jeremy Corbyn. Labour received more than 40 per cent of the popular vote in 2017 – more than any Labour leader since Tony Blair in 2001.
What was truly amazing is that he did this while he had to fight his own party every step of the way.

Corbyn won with 59.5 percent of the vote. Burnham, the runner-up, received 19 percent. According to the Guardian, “shell-shocked members of the shadow cabinet, some on the verge of tears, gathered together in small groups in the foyer” in reaction to the victory, while others “continued plotting, in the manner of Japanese soldiers who refused to surrender at the end of the second world war.”

The neoliberal Blairites failed at preventing Corbyn from becoming the leader of the Labour Party, but that doesn't mean they just gave up.

Instead, within mere hours of Corbyn’s victory, Yvette Cooper — one of his former leadership rivals — and six other shadow cabinet members declared they wouldn’t serve under him.
...His own MPs yelled “Resign” at him in the middle of a parliamentary debate. They passed a no-confidence motion against him by 172 to forty, and nearly sixty former Labour parliamentary candidates called for his resignation.
...Labour MPs’ efforts to defeat Corbyn failed a second time, however, with Corbyn beating his sole challenger, Owen Smith, by an even larger margin than in his first victory.

The neoliberals stopped at nothing to bring down their party leader. When the Tories called a snap election, thinking they were going to destroy Corbyn once and for all, the Blairites jumped on board.
Their criticism of Corbyn became absolutely bizarre and hysterical.

The Blairites are so consumed by hatred, and so incensed that anybody should offer a political alternative to neo-liberalism, that they simply cannot stop....The BBC Newsnight “policy editor” has just been tipped off, by two of the usual anonymous Blairite MPs, that Corbyn has stooped so low as actually to ask people to vote Labour, as part of his vile plot to seize power. I am not making this up.

Finally came the election and Corbyn's Labour, now with a socialist agenda, surpassed all expectations. So did the neoliberal Blairites finally admit they were wrong and give up?
Of course not. They immediately started plotting on forming a centrist party to steal moderate voters from Labour, to the benefit of the Tories.
On a local level the Blairites are working hard to undermine Labour's gains from the election.

During the last two years, however, Labour’s right wing has gone a bit further than that, trying to destablise Corbyn, and then criticising him for not being stable. Their aim is to return the party to the free market agenda of the 1990s and 2000s.
It’s understandable, then, that the left of the party hopes these local ballots will construct a bulwark of sympathetic MPs against the right. But data on the LabourList website shows that, of the 32 target seats where members have already chosen their candidate, only eight have selected an individual backed by the grassroots pro-Corbyn group Momentum.

Corbyn's experience is instructive for why the center-left is in collapse, and who are the real enemies of socialists (hint: they aren't the opposition party).
In America you saw the Democratic primaries openly rigged from the DNC down, and now with progressives fighting back, the neoliberal establishment is working hard to undermine the party's grassroots.
Corbyn showed us that the people can win against the neoliberals. But it also shows us that the neoliberals will never, ever stop. They are tools of the establishment, and that's why they are so hated.
The neoliberals, like Clintonites, will gladly implode the Democratic Party if the only alternative is a progressive victory.