In 2000, when Al Gore selected Joe Lieberman as his running-mate, Jesse Jackson decided to throw a tantrum about it, portraying Lieberman as some sort of crypto-reactionary creep. My reaction at the time was that Jackson was acting like a jerk and helping hand the election to the Republicans.
Of course I had no idea how close the election would be; as things turned out, Jackson’s move was probably the difference between having George W. Bush as President and not having him as President, which means the difference between war with Iraq and no war with Iraq. So Jackson was, indeed, acting like a jerk.
But, as Joe Lieberman has amply demonstrated, Jackson wasn’t wrong.