Most of the Democrats’ gains, however, have come from the purple states. What’s perplexing is that this has happened right as Democrats’ position on the generic congressional ballot — probably the best indicator of the national mood — has deteriorated. Historically, the generic ballot and state-by-state Senate polls — while not perfectly correlated — have moved in tandem more often than not. On average since 1990, a one-percentage-point change in the generic ballot has translated to a half-point change (in the same party’s direction) in the average Senate race.

Might Democrats be benefiting from strong voter outreach in these states — perhaps the residue of President Obama’s “ground game” in 2012? You could make that case in North Carolina, where two polls released on Monday showed a smaller gap between registered and likely voters than most other states that have been polled this year. But this story isn’t so consistent. By contrast, CNN’s poll of New Hampshire on Monday had a conspicuously large turnout gap. And in 2010, presidential swing states showed an especially large turnout drop-off for Democrats.

Money could be a more important factor. Consider the states with the largest polling movement: In North Carolina, Hagan had $8.7 million in cash on hand as of June 30 as compared with just $1.5 million for her Republican opponent, Thom Tillis. In Colorado, Udall had $5.7 million as compared with $3.4 million for Republican Cory Gardner.