A month out from Election Day, there are signs that the battleground for control of the U.S. Senate are beginning to firm up, with Republicans privately conceding they are likely to fall short of what it would take to gain control. Here´s today´s report from the insider organ The Hill:

Eight states are emerging as the battlegrounds that will decide the margin of Senate control, according to interviews with Republican and Democratic strategists.
They are Colorado, Illinois, Kentucky, Missouri, Nevada, Pennsylvania, West Virginia and Wisconsin.
Republicans need a net gain of 10 seats to win control of the chamber.

If accurate, this report suggests that the GOP has become pessimistic about the prospects of Dino Rossi in Washington and Carly Fiorina in California. Both have been drifting behind their incumbent opponents, Patty Murray and Barbara Boxer, in recent polls. It also indicates that the political fundamentals in Pennsylvania and Wisconsin are likely to tighten those two races; Pat Toomey has had a steady lead for months in PA, and Ron Johnson has recently surged into a sizable lead in WI.
With Arkansas almost certain to flip from D to R, Republicans could win all eight of those battleground contests and still wind up with a tie in the Senate, to be broken by Vice President Joe Biden (barring some unlikely deal with Joe Lieberman or Ben Nelson to gain a majority).
Estimates of Republican chances at a Senate majority that rely on polls rather than insider calculations aren´t bullish, either. Nate Silver rates the odds of that happening at 22%. But he warns:

[T]he Senate will not come easily for Republicans. But, in contrast to previous weeks, the party seems to have multiple paths toward gaining control of it: one runs through “new” states like Connecticut and West Virginia where the polling has been moving in their favor, and the other through “old” states like California and Washington where the numbers had been running against them, but the momentum could reverse itself.

That´s worth remembering, not that Democrats are in any particular danger of becoming overconfident of anything this year.