RACING THE ECONOMY:

[R]epublicans retain big leads to pick up three open seats in states carried by Mitt Romney --West Virginia, Montana and South Dakota. Republican nominees have moved ahead of three Democratic incumbents in Romney states (Alaska, Arkansas and Louisiana) and in two target states carried by President Obama (Colorado and Iowa).

Only in North Carolina, which Romney narrowly carried, has the Republican not yet overtaken the incumbent Democrat Sen. Kay Hagan -- and her edge is narrowing in the most recent polls.

Psephologists used to have a rule that incumbents running below 50 percent against lesser-known challengers would inevitably lose. Everyone knows them, the logic went, and half aren't voting for them.

That rule doesn't seem to apply anymore, but perhaps another one does. The RealClearPolitics average of recent polls puts Democratic incumbents in these five states at 41 to 44 percent of the vote.

In seriously contested races in the last six Senate cycles, starting with 2002, only two incumbents polling at that level in September ended up winning. One was appointed to an open seat and thus probably not widely known.

Both ended up with less than 50 percent and won by plurality.

Psephological rules are made to be broken, sooner or later.

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