One thing you'll hear in the media is that "the Republicans have peaked too early." Of course there's only one day that counts, election day. How a party would do before then is speculation and irrelevant. In January some were saying Scott Brown peaked too early when polls showed him ahead a week before election day. Maybe he did peak early. What mattered is that he won.
There are reasons to think Republican momentum has ceased. Rasmussen does the generic ballot every week, and their Republican lead has been cut from 12 to 6 since early September.
Senate polls in California, Washington, Kentucky, and Delaware are all more favorable to the Democrats than they were a few weeks ago.
Since September 10, Democrats have led on roughly half the congressional polls that have been publicized. Many of these polls have been internal or from the DCCC. That may mean that these polls are skewed toward positive Democratic results, but when one party is putting out polls and the other isn't, it usually means that party has something positive to say and the other doesn't. This could mean that Republicans have crested in the polls and things are turning for Democrats.
But it probably doesn't.
While Rasmussen's Republican generic ballot lead has been halved, a 6 point lead is still significant. Every other pollster using "likely voter" has seen an increase for the Republicans in September, including the new CNN/Opinion Research and GWU/Politico/Battleground polls that have come out in the last few days. Ten different pollsters have used "likely voter" this month and they average a Republican lead of 7.3 points. The GOP doesn't need a lead that big to win a lot of seats.
Senate polls in New York, Connecticut, West Virginia, Wisonsin, Pennsylvania, Ohio, Florida, and North Carolina have all taken a decidedly Republican bounce this month. 538 has Republicans at the highest seat total they've had them all year, 47.6. That's only up half a seat from a month ago, but having 47-48 seats, a pick-up of 6-7, would be a nice haul for the GOP.
While Democrats have been ahead in more than half the polls in the last two weeks or so, 45 of the 52 are in seats currently held by Democrats. When you have 45, winning 28 and losing 24 is still a net loss of 17 seats. Extrapolating those results over all the seats in play, Democrats would lose 44 seats and control of the House. If Republicans are stuck on a 44 seat net, they'll be happy. I've had them at a net +46 since Labor Day. If Republicans don't increase momentum, but do maintain it, they'll have a big day November 2. Even a slight drop off would be fine.
It's interesting, however, if the GOP wins only 4-5 senate seats and 35 House seats that'll be seen as a disappointment. For the first year of the Obama Presidency, people weren't predicting gains that big.