Through this campaign cycle it’s been treated as at least a 50-50 chance the Republicans take control of the Senate in the next Congress. It’s a mix of a few very tough retirements for the Dems and the fact that we’re six years out from the Democratic blowout in 2006. All of which leave the Democrats seriously on the defensive. And yet, pivoting against the party conventions, Republican hopes of taking over the upper chamber have dwindled dramatically over just the last couple weeks.One example I’ve been focusing on here in the Editors’ Blog is the Massachusetts Senate race between Sen. Scott Brown and Elizabeth Warren. This is a close race between two popular candidates and the lead has traded a few times. But over the last couple weeks Warren has clearly moved into the lead.

You can see the trend chart here.

(As I wrote this piece, yet another poll came out which showed Brown again in the lead. For the moment, it seems to be an outlier. Though of course it could also point to a move back in Brown’s favor.)

I’ve been figuring Warren’s move was a combination of her strong convention speech and the intensification of the national campaign in what is a deeply Democratic-leaning state. But today our PollTracker’s Assistant Polling Editor Tom Kludt pointed something out to me. It’s not just Warren who’s moving in the state. Obama is too.

No one ever thought Obama could lose Massachusetts. But he appears to have made a significant move in the last ten days. The chart is a bit jagged because this is a lightly polled state and there have been significant ups and downs. But the trend over the last couple weeks is clear.

So is Obama’s strength buoying Warren? Or is there an underlying trend driving both of their numbers?

I don’t know of any rigorous or quantifiable way of answering that question. But the fact that something similar seems to be happening in a number of states around the country points to Obama — or rather Obama’s increasing advantage over Romney — as the common denominator and driving factor.

I first noticed this when I saw that Tammy Baldwin had pulled into at least a competitive race against Tommy Thompson in Wisconsin. I’d figured this one was pretty much hopeless for the Democrats. But the momentum toward Baldwin over the last three weeks has been overwhelming.

Two polls of this race came out today. One showed it tied (Quinnipiac/CBS/NYT) and another showed Baldwin +9 (Marquette Law). The latter is the gold standard in-state poll. But both polled the race a month ago. And the movement in the first was +9 toward Baldwin and in the second a +18 move in her favor.

Nate Silver is seeing the trend too, noting that his model shows the Republican chances of taking over the Senate have fallen to a mere 21% versus over a 60% chance just four weeks ago.

There are two many variables to disentangle at this moment just what’s happening. Some moves seem fortuitous, like the Akin meltdown in Missouri and don’t have anything to do with anything beyond Akin being an awful statewide candidate. But it’s not too soon to consider whether Romney’s running stumbles — the sum of which have tended to confirm a caricature of Republicans as elitist and owned by cocooned hyper-wealth — are doing real damage to the Republican brand … damage which is seeping down ballot as Romney becomes the face of the GOP.