Actually here in the US the "politically sophisticated" voter chooses a party first, votes for their party every single time and chooses policy positions to fit their party rather than other way around.
A few percent of people just kinda blow in the wind and might on a lark join in with a presidential candidate to "have a little fun" and those people are who make the difference in competitive elections.
Having been involved those choosing the party positions often are secretly voting third party. They are trying to change the party potition and when it works the vote for the party, but when it fails they vote for who they like.
people who only vote in the general election, and maybe the primaries have choosen their party and then potitions based on the party.
In New York many elections are so lopsided that I'm inclined to vote third party because voting either D or R if I don't like the candidate is "wasting my vote".
I was active in the Green Party (twisting people's arms to run for local office) in the early 2000s and think about voting Green in each presidential election but it's been a while because I don't particularly like Jill Stein and think if you're hard on CO2 you should be easy on Uranium.
A few percent of people just kinda blow in the wind and might on a lark join in with a presidential candidate to "have a little fun" and those people are who make the difference in competitive elections.