The polling tells us that the bulk of 2014's contested Senate races are basically dogfights. So why are so many Republicans optimistic? Because it's still June, and some of the elements of the dynamics of 2014 may not be fully baked into the polling yet. How good a year this is for the GOP will depend on those factors.
If you look at the chart at the top of this post, what you pretty clearly can see from the data is that the Senate races right now seem to be sorted into three general groups (although in each group I'm including one race that is less favorable for the GOP than the rest).
Group One, three currently Democrat-held seats in deep-red territory without real incumbents, is the likely GOP blowouts. Montana and South Dakota are both looking locked up, and the South Dakota polling may get even uglier for the Democrats if the third-party support for Larry Pressler (a former Republican Senator running as an independent) fades. West Virginia is closer, close enough that a giant gaffe or scandal or something could put it back on the table, and in a different year or state a 10-point lead would not look insurmountable. But it's hard to see where that support comes from, in a 2014 midterm in West Virginia.
Group Two is the tossups, nine states that are really too close to call right now. Seven of the nine are Democrat-held seats, five with incumbents (Alaska, Arkansas, Colorado, Louisiana and North Carolina) and two open seats (Iowa and Michgigan). One of the two GOP-held seats has an incumbent (Kentucky), the other is open (Georgia). The Democrats have settled on candidates in all nine, Republicans still have a primary in Alaska (the poll average here is the matchup of frontrunner Dan Sullivan against incumbent Mark Begich), a runoff in Georgia (the poll average here is the matchup of frontrunner Jack Kingston against Democrat nominee Michelle Nunn), and a "jungle primary" that will probably result in a December runoff in Louisiana (the poll average here is the runoff matchup of frontrunner Bill Cassidy against incumbent Mary Landrieu). In only one of these races, in Michigan, does the current leader have a 5-point lead; in five of the nine races the frontrunner is below 45%, and in eight of the nine (all but Cassidy in Louisiana) below 46%. While a 2 or 3-point lead in the polls in October may be meaningful, a race with a lead that size in June and 10-20 percent undecided is functionally a tossup, at least until you take into consideration the various factors (national environment, state electorate) that are likely to pull the race in one direction or another as we enter the fall.
Why do Republican analysts feel so optimistic? Because polls, as we recall from 2010 and 2012, are only as good as their ability to project who will turn out and vote, and we are probably still a few months from pollsters being able to really make accurate assessments of what the fall electorate will look like. As Sam Wang, Ph.D., has noted, the various models for predicting how the Senate races will go are predicting different things depending on the extent to which they look beyond the polls to incorporate predictive elements like the economy, the effect of incumbency, the President's approval rating, and the like. Sean Trende, here and here, offered a model based mainly on Obama's approval rating, and found even after some tweaks to incorporate a few other variables, that Democrats could be projected to face double-digit Senate seat losses if the President's approval rating was 43% or lower on Election Day.
That's just one way of skinning this cat, but right now, Obama's approval sits at 41.5 approval/53.9 disapproval, and has been trending rather sharply downward for the past month, with his approval on the economy, foreign policy and healthcare all consistently worse than his overall approval rating. (Via Ace, it's even worse in the battleground states). In that national environment, with midterm elections in general tending to produce Republican-leaning electorates, and with the historic poor performance of second-term presidents in sixth-year midterms, you really have to feel pretty good about GOP chances of winning most of those nine races. That may seem improbable, but there were basically seven Senate races that went to the wire or involved potentially big Democratic upsets in 2012 - Pennsylvania, Ohio, Wisconsin, Virginia, Massachusetts, Indiana, and Missouri - and I didn't think at the time they would run the table and win all seven. They did. In a few of those, like Virginia and Wisconsin, the Senate races tracked almost precisely the outcome in the Presidential race, meaning turnout from the top of the ticket was decisive. If the national environment really does show as sour across the board for Democrats in November as it looks from today, eight-for-nine or nine-for-nine could be a possibility. If the environment (including the parties' turnout operations) swings back to a more neutral one, I'd be looking more at the GOP winning five of the nine, which would net a six-seat overall gain in the Senate, enough for control of the chamber but by a very narrow margin that might not last beyond 2016.
For now, that's still a big if, not reflected in polls showing voters not really ready to commit to either side in most of those races. It's why Republicans are waiting for the wave. But it's also a reminder that those races won't win themselves - Democrats ran the table in 2012 by fighting all the way to the whistle in every race with every resource they had. One thing helping the GOP may be the Governor's races: for example, Rick Snyder is now comfortably ahead in the polls in Michigan, and the Colorado GOP dodged a repeat of the 2010 trainwreck by picking Bob Beauprez over Tom Tancredo; Beauprez may not beat John Hickenlooper, but he'll give him a tough race without Tancredo's divisiveness.
Finally, there's Group Three, the races in which the polling shows the Democrats safe for now - but, depending on the national environment, maybe not safe enough just yet to declare those races over. Incumbents Jeff Merkley in Oregon, Al Franken in Minnesota, Jeanne Shaheen in New Hampshire all have leads around 10 points, and Mark Warner in Virginia has a sixteen-point lead on Ed Gillespie. (It's also always possible some other races could come on the board; there hasn't been much in the way of general election polling in Mississippi or New Mexico, for example. But we'll have to wait and see). But none of them are regularly polling above 50%, the usual rule of thumb for a safe incumbent.
Realistically, those are "reach" races that only go on the board if things really get ugly for the Democrats. Oregon is, I would guess, the best hope for the GOP relative to its present polling given the Cover Oregon fiasco, New Hampshire the toughest of the OR-MN-NH trio due to Shaheen's personal popularity and the likelihood of a landslide win for the Democrats in the Governor's race (the other two will have tight GOV races). Also, Al Franken has a huge warchest, so his race with self-funder Mike McFadden could get ugly and expensive. Virginia, of course, is the longest reach, but Gillespie should be sufficiently well-funded and anodyne to take advantage if Warner slides into the neighborhood of actually being vulnerable.
Predictions? Anybody who's predicting the fall elections in June with too much certainty is nuts. But right now, Republicans have a lot of opportunities in the Senate. If Obama's approval rating keeps tanking, the GOP avoids any major campaign-killing gaffes, and the Democrats don't come up with a magic turnout bullet, the swing in the Senate could be bigger than anyone is realistically talking about right now. Don't count your chickens; this is just the optimistic scenario. But it is not, from the vantage point of late June, an unrealistic one.