Political parties?

It's clearly implied on the party scene between Jordan and Johnny's wife, Eleanor, that they are of the same party. Jordan says clearly that he will do all he can to prevent Johnny from being nominated for VP. If he was from the opposing party, he might welcome Iselin'e nomination knowing that he had evidence to destroy him and his campaign.

I stand corrected.

In the book, Iselin and Jordan are definitely in the same party. I just inferred Jordan was in the opposite party because the Iselins referred to him as a Communist.

One thing I loved about this film was the way the ultraconservative grande dame turned out to be a secret Communist. Having lived in a region where that kind of right-wing rabblerousing is distressingly common, I enjoyed seeing it taken down a peg.

Interesting revisionist history. Since the GOP supported the Civil Rights and Voting Rights acts in higher proportion than the Democrats, why would bigoted Democratic southerners switch parties? The southern state Democrats, including Al Gore's father, voted against both acts. They desserted the party because of McGovern and then after Carter, even though he was one of their own and it was mostly over "values" issues, not race. Nixon's so called "Southern Strategy" is a myth invented to explain the shellacking the Democrats took in 1972. And Nixon himself, for all his Cold War bluster was a liberal. Who do you think started the EPA, Food Stamps, etc., in the late 60's, early 70's?

It's pretty clear they're both Republicans:
- Using Lincoln for symbolic value.
- Iselin may not exactly be Sen. McCarthy, but he's of the same ilk.
- The divide between Iselin and Jordan makes sense in the Republican party; it doesn't really in the Democratic party, since obviously neither is a southerner.

Right. The national political parties were not divided by ideology until the 20th century was more than half over. There were many Left progressives in the Republican party in the first half and especially first third of the last century. One only has to look up, among others, Robert LaFollette, Teddy Roosevelt, Hiram Johnson, Charle Evans Hughes (moderate), Wayne Morse, among many others. Liberal New York Mayor John Lindsay served in the 1960s. Pennsylvania, Wisconsin and Minnesota elected their share of moderate to liberal Republicans well into the middle of the last century, including California Governor and later Supreme Court Chief Justice Earl Warren and anti-Vietnam War Representative Pete McClusky years later.

As to the Democrats, many "race conservative" Southerners were pro-New Deal under FDR; and many were also with Roosevelt on going to war circa 1940, while Midwestern Republicans, Left and Right, tended to be isolationist even as many were also moderate to liberal economically. There were also conservative northern and western Democrats, especially in upstate New York and New England and the Far West, notably the Southwest. Neither political party was monolithic in terms of ideology till relatively recently. Pre-1900 neither party was much to the Left or Right, as the Industrial Revolution did not lead unionization and more rights and better living conditions for people of modest to medium means. Regional loyalties, and the difference between city folk and country folk were larger factors than economic issues.