Believe it or not, that seat that Heidi won hasn’t elected a Republican to it since 1958. North Dakota had a heavy Socialist influence going back to the early 20th century, which was mostly with the Republican party (the Democrat party was largely non-existent). The battles within the GOP were between the Conservative element and the Socialist Non-Partisan League. In the late ‘50s, the NPL decided to exit the GOP and merged with the weak Democrats. Federally, the Democrats dominated from 1980 until 2010, but the GOP has been largely in control of the state since after 1990.
Alas, the GOP nominated the weak at-large Congressman who was only a freshman (and polling data indicated he was having trouble throughout the campaign). Heitkamp was part of the last group of Democrats that won office statewide (and haven’t since). Just as she won, the strong GOP candidate for the open House seat took it without much difficulty. Heitkamp has to know she has a big target on her back if she goes far left (especially on guns). I expect the new GOP Congressman Kevin Cramer will probably be her opponent in 2018.
Berg was also one of the wealthiest members of Congress according to a list I recently saw.
The state is somewhat of an anamoly politically speaking. You are quite correct that its US Senators and Congressman have been mostly Democrats for the last four or five decades....which is in stark contrast to the state’s presidential politics having voted Democrat only once in the last 70 years.....a truly inpressive record of avoiding Democrat presidential candidates.
Having said that, I'm surprised she at least gave lip service matching her campaign promise to protect gun rights. We'll see.