Just as momentum was beginning to swing toward Democrats in retaining control of the Senate this November, the past 10 days — particularly the past weekend — served as a setback to those chances.

The big punch to the gut for Senate Democrats: A poll of the Iowa Senate race that showed the Republican candidate, Joni Ernst, pulling ahead to a 6-point lead over Democrat Bruce Braley. The poll, conducted by the firm Selzer & Co. for The Des Moines Register, is considered one of the most reliable in the state and the nation.

The Democrats' path toward retaining control of the Senate becomes much harder without a win in Iowa. And that reflects in the predictive models, including ones that have been more favorable to Democrats this cycle.

The Huffington Post's model, for example, now gives Republicans about a 58% chance of taking back the Senate. Pollster and professor Sam Wang's model at the Princeton Election Consortium, which has been the most favorable to Democrats' chances, now says the current conditions forecast the GOP to take control with 51 seats.

"New polls this weekend were almost entirely bad for the Democrats, who are now in danger of losing six to eight of their Senate seats while picking up — at most — one GOP seat," said Greg Valliere, the chief political strategist at Potomac Research Group. "The GOP needs a net gain of six seats to control the Senate, and that appears increasingly likely."

Joni Ernst, right, has taken the lead in Iowa polls. REUTERS/Brian Frank

The math does not look good for Democrats with 36 days until Election Day, as conditions look favorable for Republicans in eight seats currently held by Democrats — West Virginia (19-point average lead in polls), South Dakota (13.3), Montana (19), Louisiana (5), Iowa (2.2), Colorado (0.8), Arkansas (3.6), and Alaska (4.7).

Democrats need to prevent Republicans from winning six of these seats — or hope the independent candidate Greg Orman beats out the GOP incumbent Pat Roberts in Kansas and decides to caucus with Democrats. Unexpectedly, in fact, Democrats are also leading polls in North Carolina.

It's these two states — North Carolina and Kansas — that serve as the reason Nate Silver says Democrats shouldn't begin to outright panic. But if they begin to turn — as the Kansas race continues to develop and North Carolina, which features vulnerable Democratic incumbent, shifts more toward the national mood — it could mean Democrats will be in for a long Election Day.