As Mike Allen of Politico points out, this is a bit of a flip for the meta-prognosticators. Three weeks ago, Charlie Cook wondered where the GOP wave was, and two weeks ago Larry Sabato asked the same question. Thanks to continued polling erosion from Barack Obama and a sense of a leadership vacuum in Washington, voters are becoming more clear on the need for change. And so the prognostications have begun to change, with Stuart Rothenberg setting the floor for Senate switches to the GOP at seven:

While the current Rothenberg Political Report ratings don’t show it, I am now expecting a substantial Republican Senate wave in November, with a net gain of at least seven seats.

But I wouldn’t be shocked by a larger gain. …

To be sure, Pryor is much better off now than Lincoln was at this point in 2010, and Republican challengers have not “put away” any Senate races. But any Democratic incumbent sitting in the mid-40s in a very Republican state probably can’t expect to get the benefit of the doubt from voters. And that puts Democratic Senate seats in swing states like Iowa and Colorado at great risk too, especially if the GOP “breeze” that I am expecting actually appears.

With the president looking weaker and the news getting worse, Democratic candidates in difficult and competitive districts are likely to have a truly burdensome albatross around their necks.

That is why, at least right now, I expect 2014 to be a big Senate year for the GOP — even if my current ratings don’t quite show it.

Sabato updated his predictions just before Labor Day to reflect a rising wave:

The overall picture is this: A Republican Senate gain of four-to-eight seats, with a GOP Senate pickup of six-to-seven seats the likeliest outcome; a GOP gain of somewhere around a half-dozen seats in the House; and little net party change in the gubernatorial lineup even as a few incumbents lose. 

That was written before Obama’s polling took a further downward turn, too. Charlie Cook does the math, noting that the GOP candidates appear to have experienced a surge coming out of Labor Day, and might win even absent a “wave”:

One question has become more pressing as Election Day nears: Where is the Republican wave? For Democrats, the good news is that there doesn’t appear to be an overwhelming Republican tide this year; the bad news is that Democrats could well lose the Senate even without such a wave. Six of the most competitive races are Democratic-held seats in states that Mitt Romney carried by 14 points or more. With a map like that, Republicans don’t need to dominate the country; they just have to win some select states. …

Only one of the seven Democratic toss-up seats has seen any real change over time—and that’s in the GOP’s favor. In Iowa, Democratic Rep. Bruce Braley’s small lead over Republican Joni Ernst has gotten, well, smaller—to the point of basically disappearing. In Michigan, the other toss-up state where Democrats had something of an edge in late spring, Democratic Rep. Gary Peters still barely outpolls former Secretary of State Terri Lynn Land, the GOP standard-bearer. …

That leaves no fewer than nine very close races, at least half of them headed toward photo finishes. But three Democratic-held seats are already gone, and party strategists see seven more teetering on the edge, compared with just two for Republicans. Given that equation, you’d have to bet on the GOP.

This was also written before the WaPo/ABC poll, and just before the Marist spread of Senate polling that showed the GOP gaining momentum. Meanwhile, Nate Silver’s more recent update calls Republican control of the Senate the “past of least resistance,” ironically:

The bottom line is not much has changed. The FiveThirtyEight forecast model gives Republicans a 65.1 percent chance of winning the Senate with the new polling added, similar to the 63.5 percent chance that our previous forecast gave them on Friday.

But the path to a Republican majority is becoming a little clearer — and the problem for Democrats is that it runs through six deeply red states. …

The FiveThirtyEight model automatically shifts registered-voter surveys toward Republicans to make them equivalent to likely-voter polls; this is one of the reasons our forecast is slightly more favorable to Republicans than others you might encounter. It’s likely that other models will show a shift toward Republicans and come to more closely match ours as more polling firms begin to release likely voter results.

For another level of irony, try this: The long-term problem for the GOP is that red-state path. This is the result of the 2008 Democratic wave of 2008, when Barack Obama led the ticket to big wins across the board. That’s the reason Republicans have so many options to a majority in this cycle, along with the plummeting poll numbers for Obama.  In 2016, Republicans will have to defend the seats gained in the 2010 midterms, plus contend at the top of the ticket for the presidency. It’s very conceivable that the GOP may not be able to gain seats even if they do win the White House, and could very well lose ground in the Senate. If Republicans don’t win a majority in 2014, it becomes very, very difficult to do so in 2016.

A GOP win of the Senate majority may be the smart bet in 2014, but it’s a last-chance opportunity for Republicans, too. They had better make good on it, or it will be a long dry spell for them in the upper chamber.