This map, from the New Republic's Jonathan Cohn, is a good way of conceptualizing just how bad the 2014 Senate map is for Democrats. If the only states voting in 2012 were the ones voting for Senate tonight, Romney would have easily won the 2014 election:
That's the basic structure of the election — and that's before you get into the fact that Obama isn't on the ballot, and his vaunted campaign operation isn't in play, and he's significantly less popular than he was in 2012.
As Cohn writes, "it's not simply geography that's undermining Democratic strength this time around. If it were, Democrats wouldn't be struggling to hold seats in places like Iowa, which Obama won. But the electorate for this Senate race is a lot more conservative than America as a whole."
This is part of why the Democrats' message has been so muddled. At Reason, Peter Suderman notes that even if Democrats hold onto the Senate tonight, they'll have done it by running away from Obama — sometimes with ridiculous results:
Democrats in close races have spent much of this campaign attempting to distance themselves from Obama. Little more than a third of Democrats have expressed clear support for Obamacare, and several, including Kentucky Democratic Senate candidate Alison Grimes, have refused to even say who they voted for. In Colorado, Democratic Senator Mark Udall has tried to portray himself as oppositional to the White House. "Let me tell you, the White House when they look down the front lawn the last person they want to see coming is me," he said in September. In Alaska, Democratic Sen. Mark Begich declared himself a "a thorn in his [posterior]." A recent rally for Sen. Mary Landrieu (D-La.) feature a parade of speakers attempting to "distance Landrieu from President Barack Obama and instead associate her with Clinton," according to The Huffington Post.
I assure you: Mark Udall is not close to the last person the White House wants to see on their lawn.
Part of this messaging is that Obama is unpopular. But part of it is that the election is playing out in red states in which Obama is always unpopular. Democrats running in Kentucky and Louisiana in 2012 were not exactly trumpeting their close relationships with Obama, either. When the election is taking place on a battleground that's 7.2 percentage points more conservative than the nation as a whole, that's going to change the message Democrats run with.
It's possible that Democrats will deliver turnout far in excess of what the pollsters expect and surprise everyone tonight. But the basic shape of this election overwhelmingly favors Republicans.