It will be interesting to see whether President Obama can use an economic-success argument more successfully state by state, instead of nationally. Consider that there’s been a lot of mixed economic news recently. The automobile industry is booming, as are parts of the farm economy. Both are major pluses for Obama.

MSNBC’s “Morning Joe” show had a good segment Tuesday with Agriculture Secretary Tom Vilsack, who I doubt gets a lot of mainstream airtime.

That’s a shame, because he has a great story to tell about farm exports helping the economy in some states.

Of course, not all the news is good. Retail spending was weak in April, the housing market remains flat to depressed, and the European debt crisis seems held in check by chewing gum and bailing wire.

Perhaps reflecting this muddle, a USA Today poll published this week shows that Americans are more optimistic that the economy will improve, which should help Obama, but respondents believe the economy will do better under Mitt Romney.

I have a pretty simple theory about the election: If the unemployment rate is 8.4 percent or higher, Romney will win relatively easily. If it is below 7.9 percent, Obama will win. If it is somewhere between those two points, the race will be historically close and the skill of the candidates and campaigns will determine the outcome.

Now, let’s apply that theory to the battleground states. Here is a list of the RealClearPolitics.com battleground states, matched with their March unemployment numbers:

Arizona 8.6 percent

Colorado 7.8 percent

Florida 9.0 percent

Iowa 5.2 percent

Missouri 7.4 percent

New Hampshire 5.2 percent

North Carolina 9.7 percent

Ohio 7.5 percent

Virginia 5.6 percent

According to my theory, Obama wins Colorado, Iowa, Missouri, New Hampshire, Ohio and Virginia. Romney picks up Arizona, Florida and North Carolina.

Distributing these results to the RealClearPolitics electoral map model results in a victory for Obama, with 313 electoral votes to Romney’s 225.

Of course, this analysis doesn’t account for the impact that national unemployment numbers have on the psychology of all Americans, regardless of how well their particular state may be doing. But presidential elections are increasingly a series of statewide elections, as many states are already solidly blue or red. The real battle will be fought in a handful of states that are the true toss-ups. In many of these, the economy may actually work for Obama, or at least not against him.

Carter Eskew was the chief strategist for the Gore 2000 presidential campaign. This column was distributed by The Washington Post, where it first appeared.