Yes, Republicans are much more convinced that they are headed for a win on Nov. 4 than are Democrats. But, Republicans were also convinced that they were going to beat President Obama in 2012. And that didn't work out so well.
So, do expectations -- high or not -- matter? The answer is they can -- especially in a midterm election.
Remember that the key difference between a midterm and a presidential election is turnout. Think of it this way: a presidential election is like the Super Bowl -- everyone, even casual football or sports fans, pays attention. Midterms are like a middle-of-the-season game between the Tennessee Titans and Cleveland Browns; it's the sort of things that really only the diehard fans of the two teams love.
With midterms being a battle of the bases, one side being more energized matters. There's a reason that in the Pew data above the party with the high expectations was the one that won big on election day. In some ways, the expectations question is a self-fulfilling prophecy. Republicans right now believe this is going to be a good election for them. That makes them more likely to volunteer their time -- and their money -- to support the party and its candidates, and to turn out on election day. Those efforts, in turn, make it more likely that Republicans actually will have a good election on Nov. 4. It's a virtuous cycle for Republicans.
Then there is the impact that the expectations chasm has on loosely-affiliated partisans and those who are genuine independents. While their collective interest in the midterms is far lower than in a presidential, they can still sway a race that is close to start with. Human nature dictates that we all like to be on the winning team (there are A LOT of Yankee fans not from New York) -- and this goes double for political independents who, by definition, bounce between the two parties. If Republicans look like the winning side two weeks or two days before the election, it's easy to see independents throwing in their lot with the GOP.
Expectations can be a very dangerous in politics. But when it comes to midterm elections, winning the expectations game is a very good thing.