OPTIMISTIC, WITH A CHANCE OF DOUBT…. About a month ago, Mark Halperin suggested Republican gains in the House would be so significant in the midterms, a 60-seat pick-up was not at all out of the question. Last week, when Rep. Michael Burgess (R-Texas) hedged before endorsing John Boehner for Speaker, he referenced a similar number: “[Y]ou may have 60 or 65 brand new faces over here on the Republican side. I don’t think it’s fair to lock us into to just about anything right now. Let’s have the election and see what happens.”

This kind of assessment has been fairly common. Republicans need a net gain of 39 seats to take back the House majority, and the widespread assumption has been that this won’t be especially difficult for the party. The question hasn’t been whether the GOP would get to the 39; the question has been whether the first digit of their total gains would be a 4, 5, 6, or 7.

Ever so subtly, the assumptions have begun to change a bit. Last week, Chris Cillizza reported that “Democratic strategists are — quietly — growing more optimistic about their chances in the fall election.” The New York Times reported over the weekend that Republicans remain optimistic, but hoped to be in a better position four weeks out.

Republicans carry substantial advantages as they move into the final month of the fall campaign, but the resilience of vulnerable Democrats is complicating Republican efforts to lock down enough seats to capture the House and take control of the unsettled electoral battleground.

By now, Republicans had hoped to put away a first layer of Democrats and set their sights on a second tier of incumbents. But the fight for control of Congress is more fluid than it seemed at Labor Day, with Democrats mounting strong resistance in some parts of the country as they try to hold off a potential Republican wave in November.

The chances of a Republican takeover in the House remain far greater than in the Senate, according to a race-by-race analysis by The New York Times. But enough contests remain in flux that both parties head into the final four weeks of the campaign with the ability to change the dynamic before Election Day.

The NYT also reported that Republican strategists estimated that “only half of the 39 seats they need to win control of the House were definitively in hand.” Many Dem incumbents “remain vulnerable,” the report added, “but their positions have stabilized.”

It’s very hard to know, of course, about the motivations for scuttlebutt like this. Indeed, it seems more than likely that GOP sources are finally realizing that they’ve been playing the expectations game pretty badly — at this point, a net gain of 35 seats would be perceived as a tragic failure for Republicans — so they’re now making an effort to temper the outlook, keep up enthusiasm, and combat over-confidence.

In other words, it’s possible the party feels as if it’s locked up 39 seats, but they don’t want to say they’ve locked up 39 seats.

But these whispers of doubts nevertheless seem to be louder lately. The underlying factors of the cycle haven’t changed much in months, and a weak economy coupled with a riled up, right-wing activist base may yet prove to be a disaster for Dems in the midterms. But Politico reported this morning that “once-despondent Democrats now believe that they may be able to avert a total midterm wipeout, as a series of important states now appears to be trending in their direction or growing more competitive.”

Bottom line: it’s not over.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.