Democrats are on the verge of locking up several Senate races in the Northeast, including one in Connecticut that some analysts had considered a toss-up. But Republicans have gained ground overall in this week’s Senate forecast by virtue of improved polling in Nevada and West Virginia. Their odds of taking over the Senate on Nov. 2 have now improved to 24 percent — up from 22 percent last week and 15 percent three weeks ago, according to the FiveThirtyEight forecast model.

Let’s start with the good news for the Democrats, because there certainly is some of it — mostly concentrated along the Eastern Seaboard.

No fewer than five new polls have been released in Connecticut in the last week, a state where our forecasting model had long been skeptical of the chances for the Republican, Linda McMahon. Each poll shows the Democrat, state Attorney General Richard Blumenthal, with leads ranging from 7 to 13 percentage points. In addition, each of the polls also show Mr. Blumenthal with at least 52 percent of the vote, and few undecided voters remaining in the state. Under ordinary circumstances, figures like these would suggest that Mr. Blumenthal has the race all but wrapped up.

Ms. McMahon’s campaign has recently released ads critical of Mr. Blumenthal over misleading statements he made regarding his service in Vietnam, and some of the polling predates these commercials. But the polling also suggests that many voters in Connecticut feel bombarded with commercials from Ms. McMahon, who has put more than $20 million of her own money into the race, so it is doubtful the ads will be enough to allow her to overcome what is now about a 10-point deficit.

Democrats are also close to closing out the race in Delaware, with new polls showing Chris Coons with leads of 17 and 19 percentage points over his controversial opponent, Christine O’Donnell, possibly expanding what had already been a double-digit lead. Like Ms. McMahon, Ms. O’Donnell’s campaign is flush with cash, thanks to a burst of donations from conservative and Tea Party activists following her victory in the Republican primary last month. But Ms. O’Donnell’s polling deficit will be difficult to overcome and the race is perhaps best considered a curiosity rather than an integral part of the Senate picture.

Republican chances of winning the special Senate election in New York, which were never particularly strong, are also down. Their candidate there, Joseph J. DioGuiardi, whose chances improved when Carl P. Paladino made a seeming surge in the polls, may now have suffered as Mr. Paladino’s numbers have fallen back to earth. The model gives the incumbent in the race, Democrat Kirsten Gillibrand, about a 97 percent chance of winning.

These Eastern states, however — ordinarily solidly Democratic — were always something of the rainbow over the horizon for Republicans: states that might have been winnable if Democrats were to suffer losses of catastrophic proportions. But those states were not the most essential to their chances to taking over the Senate. Closer to the frontier, instead, are a set of six states — West Virginia, Nevada, Illinois, Washington, Colorado, and California — where Republican odds are notably stronger. Were Republicans to win five of the six states, they would claim control of the Senate, unless Democrats were able to engineer a rebound in a state like Wisconsin, where they now appear to be underdogs.

California is the least likely Republicans pickup; instead, the polling there has shown the Democrat, Barbara Boxer, solidifying her position in recent weeks. Still, it remains a more viable opportunity for Republicans than a state like Delaware and not one where Democrats can quite take anything for granted.

In Washington, the Democrat, Patty Murray, had also seemed to strengthen her position; several polls released last month had shown her with a lead. Since then, nonpartisan pollsters have mostly ignored the state. But the campaign of the Republican, Dino Rossi, showed itself with a 6 percentage point advantage in an internal poll it released this week.

FiveThirtyEight does not use internal polls in its Senate or gubernatorial forecasts, and they often show large divergences from nonpartisan polls — therefore, it still has Ms. Murray as the 70 percent favorite. But if public polling were to confirm that Mr. Rossi is tied or slightly ahead, that could give Republicans their clearest path to date for taking over the Senate, as they are now favored — although by slim margins — in the other four states they would need to claim the chamber.

These states include Illinois, where many voters remain undecided and the polling has stubbornly resisted moving in one or another direction, but where the model has the Republican, Mark Kirk, as a nominal, 56 percent favorite, similar to previous weeks.

These states also include Colorado. There, the polling has in fact moved slightly in the direction of the Democrat, Michael Bennet. One survey, from Public Policy Polling, in fact gives Mr. Bennet a single-point lead (Mr. Bennet’s internal polling, not used by the model, also puts him slightly ahead).  But the bulk of the evidence still suggests an advantage for the Republican, Ken Buck; the model has him as the 73 percent favorite, down from 79 percent last week.

It is in Nevada and West Virginia where Republicans have made their gains. In West Virginia, two new polls using Rasmussen Reports methodology — one released under the auspices of Fox News — give the Republican, John Raese, leads of 5 and 6 percentage points. West Virginia is in need of polling by companies other than Rasmussen — indeed, the race has yet to be surveyed by any traditional polling firm — but the model nevertheless now makes Mr. Raese a 64 percent favorite, in a reversal from last week.

Each of three polls released this week in Nevada, meanwhile, give the Republican, Sharron Angle, a small lead, ranging from 2 to 4 percentage points, over Harry Reid. Some caution is in order: two of the three polls use Rasmussen reports methodology, and some reports suggest that private polling in the state still shows Mr. Reid with an advantage. Still, Nevada had long been in a stalemate, and even if these reeds of evidence are thin, they are enough for the forecasting model to make Ms. Angle a 66 percent favorite, up from 56 percent last week.