R's and D's are all the same when you look at who owns them.

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.

Iselin and Jordan were both of the same party; there are few indications as to which party it is except that Mrs. Iselin negatively references Mr. Stevenson, presumably Adlai Stevenson, maybe indicating that they are (conservative) Democrats.

Why don't you go look up how many votes Democrat Lyndon Johnson got in the deepest of deep South states (Mississippi and Alabama) in 1964 when he won in a national landslide. It is pretty obvious it was LBJ who really lost the South for Democrats,
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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?

Mostly right, but the conservative wing of the GOP came to the forefront in 1964 with Goldwater's nomination, and was strong in 1976 as well when Reagan almost wrested the nomination away from Ford.
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Oh ye of massive ignorance:
https://en.wikipedia.org/wiki/Southern_Democrats

You are repeating a fiction, albeit an often repeated one. No deep south Democrat politicians became Republicans, not one, other than Strom Thurmond, but that was 20 years earlier.

"The Republican Party's success in the south is based entirely on playing the race card"
I'd like to see you name even one piece of legislation from the Republican Party that demonstrates their "success in the south is based entirely on playing the race card?"

"the modern South is much less poor than it was back in the thirties, which makes it easier for them to support the party of the rich"
I'll agree that the South is "less poor than it was back in the thirties," but before referring to the Republican Party as the "party of the rich," you might want to consider these 2 exit polls from the 2008 Presidential Election:
51% of the people with assets greater than $10 million voted for Obama (CBS)
52% of those making $200,000 voted for Obama (Newsweek)
Then there's the Prince & Associates survey that had two-thirds of those worth more than $30 million supporting Obama.
Finally, the USA's richest citizens, Gates and Buffett, were enthusiastic supporters of Obama.

Right. And the same way, the moderate (mostly Northeastern) wing of the Republicans didn't die in 1980, it changed sides and went over to the Democrats. My point was that there was that they stopped existing as independent wings within their own party, thus helping cement the political order of today.
And, I think it's an oversimplification to say that the Southern switch was "entirely" based on racism. It was probably the biggest motivation, but there were other factors; 1) the modern South is much less poor than it was back in the thirties, which makes it easier for them to support the party of the rich; 2) other parts of the Southern way of life also played a part. Southern reverence for military values was one (when the Democrats became associated with the anti-war movement), Southern religiosity was another one (and the focus of identity politics has been shifting from race to religion over the last few decades).
Denny Crane.

The rebellious Southern wing didn't die in the sixties. It changed sides and took over the Republican Party. The Republican Party's success in the south is based entirely on playing the race card, and most of the leadership of the Republican Party in Congress comes from the Deep South.

The parties at the time were nothing like they are today.
In the 1950s, the Democrat/Republican aisle wasn't an Iron Curtain like it is today, and politicians didn't have to agree with their party all the time. There was a lot more diversity within the parties themselves, so it wouldn't be unusual for two politicians from the same side (i.e. Jordan and Iselin) to hate each other more than they hated people in the other party.
For example, the race issue divided the Democrats between a dominant Northern wing which joined forces with the Republicans, and a rebellious Southern wing that was on its own until it died in the sixties. On the other side; the welfare state divided the Republicans between a dominant moderate wing which joined forces with the Democrats, and a conservative wing which essentially stayed buried before returning to power in 1980.
Denny Crane.

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.

Iselin and Jordan are BOTH Republicans.
The Republican Party in the 1950s was nothing like the Republican Party of today. It included both liberals and conservatives. The GOP's tilt toward the extreme right began in the 1960s with the mass defection of Southern segregationists from the Democratic Party in opposition to LBJ's civil rights legislation. By the 1980s the Republican Party's leadership was almost entirely from the Deep South, and the old, liberal Republican guard had virtually disappeared.

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.

I'm pretty sure that the two parties have become the opposite of what they once were. The Republican party being a conservative party would have been a liberal party at the time of the film.

More to the point, Jordan would be in no position to keep Iselin from the VP nomination if he was in the opposite party.
Jordon is clearly modeled after New York Republicans of the era, such as Jacob Javits, who today would probably be considered too liberal even for the Democrats.

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.

Uh, where'd you get the idea that Iselin's a Democrat and Jordan's a Republican? I don't think they ever say. My personal theory is that all the characters are Republicans, some just don't like each other.