Pew Research
Bottom line: comparing cell phone plus land line polls, to land line only polls, results in a bias in favor of Republicans of four to six percent.
The TPM poll tracker gives Republicans a lead of 6.3% in the generic Congresional ballot.
Nate Silver's 538 shows 4 Senate races [ CO, NV, WV, IL ] where the Republican candidate leads in his composite poll by fewer than 4 points. Johnson in Wisconsin leads by 6.
A little further out, there are 3 races at R+7: [ PA, KY, NH ].
What is not known is how many of the polls in the TPM / 538 composites include cell phones versus being land line only. Those polls (e.g., Rasmussen) that use auto-dialers cannot poll cell phones, which may account in part for the oft-cited Rasmussen Republican bias.
Nonetheless, this is a favorable sign for Democrats.
So, in conclusion:
GOTV. The Republicans will outspend us (thanks for Corporations Untied, SCOTUS!), but don't let them outwork us!