Michael Dobie Michael Dobie is a member of the Newsday

Michael Dobie is a member of the Newsday editorial board.

PHILADELPHIA - It’s no surprise when Bernie Sanders’ supporters boo at one of his rallies. The surprise on Monday afternoon was that they were booing . . . Sanders himself.

OK, let’s be fair. They actually jeered long and loud when he exhorted the crowd in a ballroom at the Pennsylvania Convention Center to vote for Hillary Clinton in November. But these folks were in no mood to be receptive to that plea.

They had come by the many-many-hundreds for a sort of pre-game warmup, to hear Sanders speak in advance of his formal address to the Democratic National Convention later in the evening.

Some were carrying signs, some were wearing green felt Robin Hood hats, but all of them were stoked.

They likely would have been anyway, but the drop of hacked emails from the Democratic National Committee and the subsequent resignation of chairwoman Debbie Wasserman Schultz added, shall we say, a little more energy.

And a little more interest than perhaps was anticipated. The number of people who wanted in to Sanders’ speech was too large for the huge ballroom so organizers shut off access before he took the stage. But in a move that seemed somewhat un-Sandersesque, they also closed the doors so no one outside could hear.

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And they stayed closed, despite one would-be attendee’s inspired try: “Candygram.”

Those who hung around, ears close to cracks in doors or peering around plants through a window in the wall, were treated to a triumphant Sanders boast when he came out shortly after 1 p.m.

“Make no mistake about it: We have made history,” Sanders shouted. “A year has come and gone and we’re not fringe players anymore.”

But the resounding cheer that greeted that proclamation — and the wild applause that followed his delivery of the news that Wasserman Schultz had decided not to gavel in the convention — were followed shortly by those disgruntled boos — another log in the fire of people who feel that they were cheated.

All of which confirms the growing belief that uniting these Democrats is going to be more difficult than the party expected.