With one robocall to African American voters, thad cochran was able to swing that primary election by getting thousands to the polls that otherwise wouldn't have showed up. If an old white Mississippi republican can do that with a robocall, I see no reason why democrats can't get these same folks to the polls on November 4th.
Because he was running against an extremist. Not a single one of the republican candidates in competitive races (not even Ernst or Cotton) is like McDaniel.
Secondly, midterm turnout is always lower, and more republican, than presidential elections. Simple fact that proves true every single time. With the exception of 1998, the president's party always loses seats in both houses of congress in their sixth year. It's something we've seen happen under the beloved Ronald Reagan, the hated George W. Bush, and tons of other presidents.
When it's just a few seats in the senate and not the presidency, people don't care, and no amount of robocalls is going to change the "It doesn't matter how things turn out. Filibusters/teapartyists/far-leftists/reid/mcconnell etc. will prevent anything meaningful from getting done, and we aren't choosing a world leader who will attempt to bring on a new agenda." attitude.
Furthermore, in 2010, the polls in CO and NV were wrong because on-the-fence voters couldn't bring themselves to vote for Buck/Angle. Other competitive races where republicans were leading in polling - WI, PA, MO, KY, NC, IL - still went republican. The only race where that sort of last minute fallout could happen this time is in Iowa, and even that's doubtful. Cotton might qualify if he were running in NC or CO, but he's running in Arkansas, which he is definitely not "too far right" for. The non-IA competitive races where republicans lead (AR, LA, KY, GA (at least once we get to a runoff), CO, AK, SD), if things stay as they are for the next two weeks, will go republican.
The best reasonable scenario for the democrats at this point is 51-49, and even that's doubtful.
