They're centrists, scared centrists, who recognize that political in-fighting can result in a split vote letting the party with less votes take the election. However in addition to being humongous pricks about it (helping the in-fighting), they fail to recognize two very obvious facts:

Progressives don't have a candidate. In any party. You can't split the vote if there's no other option to split the vote with.
The popular vote doesn't matter. Hasn't mattered since the Republican party stole the election in 2000 (if it ever mattered in the first place). There's somewhere between 3-6 states where the popular vote, if you trust it to be recorded and reported accurately, can swing the election one way or another. In the other 44 states, the popular vote does. Not. Matter.