I see roughly three different, overlapping strains at work:

1. Moralistic pacifism: for most of the past century, committed pacifists have been mostly in the Democratic Party. For the first half of that period, they were an exceedingly small fringe (Quakers and the occasional Jeannette Rankin) but their numbers flared (like the hems of bell-bottoms) with the flower children and McGovernites of the 1960s/70s. The newer types were not so much morally rooted as reflexively opposed to all things military. In 1993 the Clintons (themselves warmed-over flower children) brought a few such types into the White House; you may recall reports of Clinton staffers being "upset" that there were military aides in the White House -- in uniforms, no less! One can respect the moral austerity ("plainness", if you will) of Quaker pacifism, but the flower-child version of it is pie-eyed dopiness (or perhaps dope-eyed piousness).

2. Nationalist isolationism: a strain rooted not in morality but in venal selfishness. In the 1900s and 1910s, the William Jennings Bryan-style Democrats blended this strain with #1. From about 1920 to about 1950, it was a core tenet of the Republican Party, much to our nation's disservice and the world's regret. The two parties seemed to reverse roles in the 1970s, so that by 1992 the Democratic Party was unabashedly running on a platform of "It's the economy, stupid" and mocking George Bush for "wasting" time on international relations. The best thing about this particular strain is that, because it is based on selfishness more than anything it else, the party temporarily embracing it can be quick to discard it when (as inevitably happens) events prove it to be folly.

3. Communist sympathism: This mental failing has been exclusively the province of the left, and therefore of the Democratic Party, and is the hardest to understand. It is not rooted in morality, for what morality would exalt tyranny and murder? It is not rooted in selfishness, for what self-interest would exalt the enemies of our own civilization? As the graves of hundreds of millions bear witness, it can arise only from willing, intentional, inhuman blindness. The century-long refusal of Western leftists to recognize communism as murder, and their readiness instead to make factless excuses for it and to enamor themselves of Communist "heroes", is one of the most bizarre and inexcusable themes of modern political history.

Summary:
- Pacifism: mostly Democratic, but not always important
- Isolationism: alternatively embraced and discarded by Republicans and Democrats
- Communist sympathism: exclusively Democratic, and the most consistent and wrongest strain of all