There are no political parties
At least, not at the same level than in other countries.
With first past the post, party discipline is way looser than it could be in, say, Germany or Spain, where party central organization has a lot of say in who runs and in which position, thus altering their probabilities of getting elected. Thas ensures that elected charges stay inside the party line.
For example, I remember a question several years ago about a neonazi running for election for the Republican party, even if the Republican party leadership was strongly against it. He got to win the local primaries(*) so he got to run for that party; all that the party could do was to publicly disavow him and provide no funds for his campaign.
The downfall for this is that the parties are more a collection of individual personalities (albeit closely aligned) than a monolithic block, and the support of a party does not guarantee the votes from all the members of that party (not the POTUS, but see what happened to Mitch McConell).
The POTUS has no party
Ok, he was voted on a platform supported by a party.
But because of the above point, he cannot be 100% sure of the support of the members of his own party. And he cannot discout the support of members of the opposing parties. Regularly, propositions from the POTUS get voted against members of his own party, or get bipartisan support.
That would be unheard of in Europe; if a PM had his party voting against him it would shortly lead to either a dissolution of the parliament and new elections, or (most likely) his dimission.
(*) IIRC it was in a NY district that was always taken by Democrats, so there was no internal competition for the position.