New York Times: “Much of what influences third-party candidate support isn’t just a straightforward desire to see that person become president. This poses a challenge for pollsters no matter what they do: Simply by listing third-party candidates, a poll might overstate their support. If a poll doesn’t list them, however, it can’t capture their support at all.
”“This year, to combat that concern, many reputable pollsters ask both versions of the question: one that poses a simple head-to-head contest between major-party candidates, and one that includes third-party candidates who may be on the ballot.”
“And which question gets asked first is where the difference comes in.”