Republicans are now a nearly two-to-one favorite to take the Senate

Two-to-one: not quite what I'd call betting odds. Republicans don't actually have a 66% chance of taking the Senate, but the 64% chance they have in our latest forecast is closer to it than any other forecast we've done since we'd started. Previously Republicans consistently had a slightly better than even chance of winning, usually in the mid-50s. The lowest they had gone was actually slightly worse-than-even chances of taking the Senate:

That was September 20. These were the days when Marks Udall and Begich were favored in their races, and Mark Pryor was at a pretty even chance to keep his own. Despite Michelle Nunn having a less than one-third shot at coming out on top in Georgia and Gary Peters being only a 3-to-1 favorite in Michigan as opposed to the near-certain victory he has now, this was the peak for Democratic chances because:

1. This was the first forecast where Greg Orman was favored above Pat Roberts in Kansas because two days prior the Kansas Supreme Court ruled that Democrat Chad Taylor's name could be removed from the ballot. As a result, we removed all polls with Chad Taylor's name in them; the ones remaining all had Orman up, and thus the independent's chances shot up from about 20% previously to almost 60%.
2. Michelle Nunn was down substantially, and Gary Peters wasn't doing as well as he would later, but it didn't matter because Democrats had so many good chances in the middle five states that those two states didn't matter as much anymore. Democrats simply had as many reasonable paths to 50 seats as Republicans did to 51.

Since then, however, chances have only declined for Democrats in Colorado, Iowa, North Carolina, New Hampshire, Alaska, and Arkansas. Greg Orman is now more likely than not trailing, if only slightly. They've consolidated leads in Michigan and pulled back into the game in Georgia, but they've just taken so many losses in the closer races that they have definitely slipped behind.

In large part this decline is due to the most dramatic fall in Democratic chances, which took place in Arkansas. Our model, unlike others, gave Pryor almost even chances to retain his seat because it places a good amount of faith in his incumbent status and his family name (his father, David Pryor, served for four years as governor and eighteen years as a U.S. senator in the very seat Mark Pryor is trying to hold on to). However, with two polls out showing Pryor well behind--one from Issues & Answers (I really hate uncreative names) and one from Rasmussen (which hasn't so far displayed the Republican house effect it has exhibited in previous years)--those advantages don't seem to be helping him much. Or if they are, they're only a mild cushion against an even more dramatic defeat: our model predicts him to lose by about four points (although neither he nor Rep. Tom Cotton will break 50%), with only a 20% chance of victory in the state that elected him unopposed by a Republican in 2008.

Pryor's dramatic decline appears to be the main driver of Democrats' overall decline. Setting his chance of victory back to 33% (which is what it was in the previous day's forecast) and Democrats' chances improve by four points to 40%. This is probably because in no other state have Democratic chances declined so dramatically in a single day. And with only four days before Tuesday, Pryor has essentially no time to make up for that slide, which is why he's four-to-one to lose. Now those are what I'd call betting odds.