Why was Vermont such a heavily Republican state for so long?

[Calthrina950]

The question is as in the title. Between 1856-1964, Vermont was the most Republican state in the country. Even before the Republican Party's formation, Vermont never voted Democratic since that Party was founded in 1828, going for the National Republicans, Whigs, and the Anti-Masonic Party. And during the century of Republican dominance, it was routinely the best state for Republican presidential candidates. Fremont, Lincoln, Grant, McKinley, T.R. Roosevelt, Taft, Harding, Coolidge, and Eisenhower all got more than 70% of the vote there (McKinley actually broke 80% there in 1896); Hayes, Garfield, Harrison, Hughes, Hoover, and Dewey got more than 60% there; and it was one of only two states to never vote for FDR.

Then, in 1964, Lyndon Johnson became the first Democrat to ever win the state, and he did so with 66% of the vote. Vermont became solidly Democratic for good in 1992, and is now one of the most Democratic states in the country. What changed in Vermont to make it transition from a Republican bastion into a Democratic one? And why were its Republican loyalties so fierce and so deep for so long?