James Carville and Democratic pollster Stanley Greenberg regularly conduct the Battleground Survey. It’s a national poll, but gives special attention to a pre-selected list of battleground states. Their latest poll confirms what many others are finding: a narrow McCain lead:

The latest events in the presidential campaign have tightened the race dramatically. In Democracy Corps’ latest surveys of 1,000 likely voters nationally and 1,017 likely voters in the presidential battleground states, the vote margin has shifted 7 points towards John McCain nationally and 9 points in the battleground. This swing puts McCain ahead of Barack Obama by 2 points nationally (48 to 46 percent), consistent with the national public polls, and by 1 point in the battleground states (48 to 47 percent).

Though these results are challenging, this is still an election that Obama has at least an even chance of winning. In the battleground states, he is running 3 points ahead of Kerry’s performance in 2004, and thus, he is very much in the position to put together the majority he needs to win the Electoral College. Moreover, the Voter Choice Scale (constructed from eight survey questions) shows nofurther trend for McCain; indeed, among independents nationally, there are nearly twice as many voters “winnable” or “in-reach” for Obama (17 to 9 percent). And finally, the shift in the party composition of the survey following the Republican convention (from an average of 8.5 points Democratic advantage throughout this year to a smaller 3-point lead last week) is very likely to ease back — probably putting the two candidates into a dead-heat soon.

That last point is probably a critical one: Carville and Greenberg regard the Republican surge in party identification as a bounce and not a bump. Without explaining why, they assume that the partisan breakdown of voters on election day will look more like what it was a few months ago than it is today. And as liberal blogger Jerome Armstrong has pointed out, more liberal than any other election year in this decade — including 2006 — the Democrats’ best year in decades. Is that really very likely? It seems equally likely that the Democratic edge in party id will erode further as the election grows closer — and as McCain more and more replaces George Bush as the face of the GOP.

Lost in the depth of the survey is another nugget that should cheer Republicans: in battleground districts with an incumbent Republican Congressman, the Republican candidate for Congress is favored over the Democrat by a margin of 58%-36%. It’s inconceivable that Democrats could realize the double-digit House gains they’ve projected, when they trail so badly in battleground districts.

Carville and Greenberg concede that the environment is ‘challenging.’ That certainly captures it.