There are only @ 750k registered voters in Montana; 538 is a decent sample size @ 98% CON, 5% M/E.
Tester is toast.
Margin of error is not determined by the percentage of voters polled. The number of people who are surveyed is the key determinant.
According to the Montana Secretary of State, there were 468,326 registered voters in the state in 2022 at the general election. That’s with a population of 1.03M.
For comparison, Kansas has 1,975,627 registered voters for a population of 2,944,376.
I’m not sure the sample size isn’t sufficient.