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Just days before the 2008 Democratic presidential caucuses in Iowa, Hillary Clinton led Barack Obama by four points in the RealClearPolitics average of polls. Obama took the lead, by a few tenths of one percentage point, literally 24 hours before the caucuses. And then he won by nearly eight points.

In retrospect, things were moving very fast in Iowa in those final days before the voting. And now, we are again days away from the Iowa Democratic caucuses. Things might be moving in ways we don't know.

Four candidates have held the lead at various times in the Iowa race. Joe Biden led from the start until the third week in September last year. Then Elizabeth Warren took over and led until mid-November. Then it was Pete Buttigieg's turn; he led until the first week in January.

The new year brought new volatility. First Bernie Sanders led until mid-January. Then Biden led again until Jan. 24. And since then, Sanders has jumped back into the lead.

In the last few days, two candidates -- Sanders and Biden -- seem to be headed up, while the other two -- Warren and Buttigieg -- seem to be headed down. In any event, this seems to be an Iowa caucus in which there is great possibility for last-minute shifts in voter choices. So predictions are useless.

What is undeniable is Democratic establishment nervousness about Sanders. The basic worry is that he is so far to the left that the party might end up nominating the only candidate President Donald Trump can beat.

Just look at some headlines from the last few days:

-- "Dems tormented over how to stop Bernie" (Politico)

-- "'Oh my God, Sanders can win': Democrats grapple with Bernie surge in Iowa" (NBC News)

-- "Worried Democratic operatives scramble to fund a network to take down Bernie Sanders" (Daily Beast)

-- "Fear of Sanders win growing among Democratic establishment" (Associated Press)

-- "'Stop Sanders' Democrats are agonizing over his momentum" (The New York Times)

Actually, that last headline, from the Times, was from April 2019. Democrats were agonizing then, and they're still agonizing now.

It is hard to imagine two more different men than Bernie Sanders and Donald Trump, but establishment Democrats fear that something could be happening in their party that echoes the events of 2016 for Republicans: A candidate with popular appeal who is dismissed by party leaders yet manages to build so much support that the establishment cannot deny him the nomination.

It might seem a little odd that Democrats are so worried by a Trump 2016 scenario. After all, Trump won. But it was, of course, a rocky road, and more than a little unpleasant for GOP bigwigs.

So now Democrats listen when the loser of that race, Hillary Clinton, expresses her apparently undying disdain for Sanders. In an admiring -- aren't they all? -- new documentary on her losing campaign, Clinton says of Sanders: "He was in Congress for years. He had one senator support him. Nobody likes him, nobody wants to work with him, he got nothing done. He was a career politician. It's all just baloney, and I feel so bad that people got sucked into it."

In a promotional interview, the Hollywood Reporter asked Clinton if that assessment still holds. "Yes, it does," she answered.

The only problem for establishment figures like Clinton is that Sanders is leading in Iowa. He is leading in New Hampshire. He is in second place in South Carolina. And then, looking down the road a bit, he is leading in the biggest state of all, California.

The establishment has reason to be worried.

Of course Sanders is far to the left. But there is no doubt the entire Democratic party has moved in his direction in the last four years. There's no indication that movement will stop. The party has come to Bernie, not the other way around. Party leaders might have to come to Bernie, too.

One irrefutable argument the party establishment would have against Sanders is that he is simply too old to be president. He will be 79 years old on Inauguration Day 2021 and will turn 80 that same year. That is unprecedented in American history. And, of course, Sanders suffered a heart attack last October.

Trump, the nation's oldest president ever, could serve four more years, and at the end of two full terms still be younger than Sanders would be on the first day of his presidency.

All that would be an effective argument, except that the establishment's preferred candidate, by the same measure, is also too old to be president. Biden will be 78 on Inauguration Day, and to some observers' eyes looks less energetic and resilient than Sanders.

So, at the moment, the Democratic establishment has no good strategy for dealing with its Sanders problem. In a week, the Iowa caucuses will be over, and the problem could be much, much worse.

Byron York is chief political correspondent for The Washington Examiner.