There are, I believe, two factors that explain the Democrat ad hominem strategy against their opponents in the health care debate (not that there's much argumentation of ideas going on). One factor is general, and the other more specific.
The general cause is the decline of reason in modern philosophy, and its effect on the left. The postmodern left does not believe that there is reason, but only subjective narratives determined mainly by ethnicity and sex. Language to the left is not used by reason to persuade, but is a weapon used to gain power. Language is a form of force.
This is why the left hates advertising so much; they think it is the way corporations manipulate the minds of the masses and make them act in ways against their own self-interest (in other words, corporate propaganda turns the innocent into right-wingers). One of Obama's first acts when he took over GM was to cut their advertising budget.
Since words are weapons, the left uses them as such. They tend naturally toward character smears and the "politics of personal destruction." So they're going after their opponents on health care the only way they really know how, by calling them a "mob," "right-wing extremists," "racists," "KKK," and I've probably missed a few choice epithets.
This is why Obama said,
"But I don't want the folks that created the mess -- I don't want the folks who created the mess to do a lot of talking...I don't mind cleaning up after them, but don't do a lot of talking."
Toleration used to be valued on the left, but those days are over. When words are weapons, then talking is force. You don't want the enemy to "do a lot of talking."
The more specific cause is the Swiftboat advertising campaign against John Kerry in 2004, an event that seems to have traumatized the left. Recently Senator Franken -- no, I can't believe he's a Senator either -- was nasty to Warren Buffet about Buffet's support of the Swiftboaters. The Democrats still remember it well, and it still bothers them.
As the left views words as weapons, it concluded from the Swiftboaters that the mean-spirited right is really good at using words to attack their side. I would say the Swiftboat attack devastated Kerry because it hit home with the truth. I think it was Aquinas who said that the most powerful argument in a debate is the truth. This doesn't even register with the left. There are no absolutes, no reason, no truth.
The left concluded that Kerry's problem was that he let the Swiftboaters sink him without responding. It has become dogma among the Democrats that when the Republicans attack, you attack back hard and fast. Thus Obama aides said they would “punch back twice as hard” against their critics. Now, even though 70% of independents are against the health care bill in the House -- which means a majority of voters -- the Democrats don't care, they're fighting back to avoid John Kerry's failure.
Modern philosophy has corrupted our culture and created this nastiness. The left sees words as weapons, and probably some precincts of the right, who are not altogether immune from the dominant trend in philosophy, also have no confidence in or understanding of reason. When people use words as weapons in the pursuit of power, without regard to the truth, can the use of real weapons be far behind? How long until factions begin shooting at one another?
UPDATE: Corrected a name. Al Franken feuded with T. Boone Pickens, not Warren Buffet, as I first wrote.