A head to head comparison of the 2020 Primary gives us an idea of the break down of the voters.
Donald Trump received 95,289 (33%) votes to beat Joe Biden who received 68,613 (24%) votes and Bernie Sanders who received 66,389 (23%) votes.
In the Primary, Trump won 400 counties in the 4-way match up. Biden trailed behind Trump winning 58 counties. Sanders won 39 counties with Warren winning only one county in Maine Primary.
Bernie Sanders vs Joe Biden
Joe Biden vs Donald Trump
Donald Trump vs Joe Biden
As for the head to head match-ups. Biden beat Sanders with 51% to 49% of the votes, and Trump also beat Sanders with of 59% vs 41% of the votes. Trump beat Biden with a margin of 58% to 42% of the votes.
These 10 counties play an important role in these Primary elections. If a candidate, can get a sizable majority in the larger counties (Cumberland, York and Penobscot), it becomes very difficult to win majorities in the smaller counties without voter outreach and increased voter participation in them
Many counties, including Lewiston, Auburn and Gorham voted in favor of Trump, giving him over 3k vote advantage over Biden and Sanders in those counties. Also, Trump's steady win in the more dense counties contributed to the vote advantage that helped him gain an upper hand in the State of Maine.
Related References: 2016 Maine General Election Story, 2016 Maine Primary Election Story