Not the most compelling threat, I’m afraid…
Arthur Silber points out that if the Democrats embrace the Hollywood! Is! Evil! brigade, and all of the hypocritical morality crusades that much of the H!I!E! crowd backs (I lived in the Los Angeles area for several years (and, oddly enough, really liked the city of Los Angeles, even though I hated living there), and after interviewing with various movie studios who tried to impress me with claims that I! Could! Meet! The! Stars!, so I've got some sympathy for H!I!E!, but not one bit of sympathy for the morality crusades), that he (and, I presume, the few other libertarians who aren't actually Evil Party members in disguise) will drop the wartime alliance with the Democrats like a hot, rotten potato.
But if the Democrats tried that stunt, the libertarian exodus (which would, regrettably, be very small, because most of the libertarians in the United States sold their souls to the Evil Party for a pottage of "business reforms") would be buried under a larger exodus of the civil liberties left who were willing to sell their souls for "business reforms" lite(tm), but only as long as the Democratic Party didn't sell out their civil liberties. Lose the civil liberties, and goodbye to a large chunk of your base; it's not 2000 anymore, the Green/Evil Party coalition is not really functional anymore, and if the party is going to lose anyway it's not going to be very compelling for the civil-liberties-are-worth-"business reforms" people to stick around to watch the B*sh junta steal it again.
The people who want to move the Democrats farther to the right don't want the libertarians either, because that pesky civil liberties thing doesn't go over very well with the bigots the GOP-lite(tm) crowd is (still! One would think that 10 years of moving to the right and losing more and more ground might teach them that appeasement doesn't work, but, no, this is the Stupid Party we're talking about) trying to seduce. The GOP-lite(tm) crowd doesn't want civil liberties, and that's all those pesky libertarians can bring to the table. So, alas, losing the libertarians isn't much of a threat to them.