Have you ever taken a look at the voter turnout for those kinds of elections?
| Candidate | Congressional District | Population of CD | Votes cast (2014) | % for GOP |
|---|---|---|---|---|
| Bradley Byrne | Alabama 1st | 635,300 (2000) | 152,234 | 68.16 |
| Martha Roby | Alabama 2nd | 635,300 (2000) | 167,952 | 67.34 |
| Mike D. Rogers | Alabama 3rd | 635,300 (2000) | 156,620 | 66.12 |
| Mo Brooks | Alabama 5th | 696,690 (2010) | 154,974 | 74.4 |
| Gary Palmer | Alabama 6th | 635,300 (2000) | 178,449 | 76.18 |
| Alan Nunnelee | Mississippi 1st | 762,914 (2006) | 151,111 | 67.91 |
| Gregg Harper | Mississippi 3rd | 711,115 (2000) | 170,946 | 68.89 |
| Steven Palazzo | Mississippi 4th | 711,219 (2000) | 155,576 | 69.92 |
These are the Republican strongholds, voter turnout appears to be somewhere between 20% and 35% (which isn't any different anywhere else in the country) depending on how many actually are able to vote age-wise and from those about 2/3 or 3/4 are voting Republican at best. So these district are nowhere even close to being "full of people who want them to vote against pretty much anything Sanders likes" - quite possibly the other way around, they appear to be full of people who've given up because they have been convinced they don't stand a chance against the Republican onslaught.
If you double the amount of people who bother voting - which happens by showing them these numbers, running a strong progressive candidate in every single district and telling people that they are not just voting for a Seat in the House but for the future of the country, I think enough people can actually get motivated enough to try or even to help and campaign for the candidates that would help.