NC isn't worth pursuing for Democrats
There is a Senate seat up for grabs. Burr has a lousy approval rating.
Who doesn't have a lousy approval rating? There's better Senate seats up for grabs.
Election rule of thumb: When you have many seat opportunities available, don't waste your time dislodging the guy up double-digits in polling. The GOP failed at this badly in 2012.
In which races exactly did they do this? CT doesn't work, because McMahon was actually polling well early in the campaign, she just faltered late. Smith getting as close as he did in PA was more because of Casey's utter refusal to campaign at all until October more than a huge republican spending effort. Kind of like the 2012 version of Gillespie, just not as strong. Bill Nelson overperformed the RCP average - by 7 (!) points - on election day, so spending money there was more or less an unavoidable mistake. The polls in MO were notoriously inaccurate, Claire McCaskill overperformed the average by 10 (!) points on election day, so again, an unavoidable mistake. So, that leaves only NM and HI where the republicans made a serious effort in a state where they were trailing by double digits in the polling average, and I'm not even sure if they were even spending in the latter of those two states in the closing month(s). So, a pretty isolated mistake rather than something that occured across the board. In any case, divert money away from NM/HI, and what do you get that you wouldn't otherwise? Probably ND and nothing else. 1 more seat doesn't really make that much of a difference in the grand scheme of things.
Maybe not double-digits, but the GOP wasted money on a lot of dumb races, like Hawaii and Michigan when it should've spent way more time in ND/MT/IN.