I'll just quote these all at once for ease of responding.
You must win Salt Lake county by truly a lot. Only in an extremely strong wave probably...
I
actually created a story on Daily Kos talking about this.
Basically, you need to match Jim Matheson's numbers in all the counties, maybe a little bit more assuming there's no Libertarian to get you right over the edge of victory.
You need about 52-54% in Salt Lake County (doable), roughly 27-29% in Utah County (more difficult, but also doable), and somewhere around 30% in Juab and Sanpete counties, which are sparsely populated, so I would just try and crank up the numbers in Salt Lake and Utah counties.
Actually, Democrats nearly held this seat in 2014, hardly a Democratic wave.
Indeed, Doug Owens got about 5% less than he needed in all of the counties in the district and he ended up losing by just under 5%. So it's totally doable, especially if you have won an election in the county before.
How did Jim Matheson pull it off in Utah?
By being a popular, well-known, moderate Congressman with a famous name, with the added benefit of a slightly stronger Libertarian than usual. As I said, he got just the right amount in every county, though I'm convinced that a Salt Lake County elected official could replicate or exceed those numbers in Salt Lake, and therefore win in the most populous county in the district.