Also, although it has become easier — perhaps more so than ever — to envision a Republican path toward taking over the Senate, they still remain underdogs and have, in essence, two significant hurdles to clear. The first hurdle is purely statistical: polls are relatively fuzzy instruments prior to the Labor Day holiday, before many voters have engaged with the campaigns. And in the weeks immediately leading up to Labor Day — when many Americans are on vacation — they can be especially erratic. If, in two or three weeks’ time, the polls continue to show clear signs of Republican momentum, their prospects for a Senate takeover will be more robust.

The other hurdle is more tangible: there are a series of competitive Republican primaries, in which candidates supported by the Tea Party are pitted against more moderate alternatives.

In Delaware, for instance, Christine O’Donnell — who has the Tea Party’s support — could defeat Mike Castle in the primary there next week. Mr. Castle, Delaware’s former Governor and now its At-Large Representative to the U.S. House, would be a clear favorite against his Democratic opponent, Chris Coons. But Mr. Coons would be favored to defeat Ms. O’Donnell, making Republicans’ prospects of a Senate takeover considerably more remote.