How did Senate races in Georgia, New Hampshire become too close to call?

The battle for control of the US Senate is getting messier thanks to tightening races in Georgia and New Hampshire.

New polls show an underdog Democrat in the South and an underdog Republican in New England close on the heels of the favorites in the two states.

These races have become tighter since September, the polls find, even as more than half a dozen other Senate races are too close to call. For now at least, uncertainty seems to be widening rather than diminishing as Election Day gets closer.

In Georgia, a poll taken from Oct. 2 to 6 finds Democrat Michelle Nunn trailing Republican David Perdue by just one percentage point, whereas most September polls showed Mr. Perdue with a lead of three points or more. The two are competing for an open seat, vacated by GOP Sen. Saxby Chambliss.

In New Hampshire, where incumbent Sen. Jeanne Shaheen (D) was leading in one late-September poll by seven points and in another by 10, the most recent polls show the spread at a narrower two, three, or six points (depending on the poll). She’s being challenged by former US Sen. Scott Brown (R), who has migrated to the Granite State from liberal Massachusetts in search of a political reboot.

Republicans need a net gain of six seats to take charge of the Senate. A Brown upset in New Hampshire could count as one of those seats.

Or, if Senator Shaheen holds firm in New Hampshire and Ms. Nunn overtakes Perdue, Republicans would have a steeper climb, needing to gain seven seats rather than six.

What has happened to make these two states more competitive?

Nunn may be gaining some traction by playing up Perdue’s role in sending US manufacturing jobs overseas during his business career. And Mr. Brown has blasted Shaheen as soft on national defense – and in alignment with President Obama on that score – while making his own name better known in door-to-door campaigning.

If the underdogs have shown some new strength, though, another factor may simply be the inherent variability in polling. It’s hard to know which polls best reflect the people who will actually vote on Nov. 4.

In Georgia, for instance, polling by WSB-TV/Landmark found Nunn to be leading in August and September (and now tied with Perdue in October), when most other polls have shown Perdue as the consistent favorite.

But another Georgia poll, by SurveyUSA, shows the race tightening from a nine-point spread for Perdue in August to just one point in late September and early October.

The wild card – something very hard for pollsters to guess in a year when voters generally are not very engaged – is which party will do better at mobilizing people to cast ballots.

The pollsters try to guess who is a “likely voter,” but until Election Day we won’t know for sure.

For now, Perdue and Shaheen remain favorites.

Shaheen is a former governor who has solid popularity with Granite State voters, while Brown seems to be losing fans (his “unfavorable” rating jumped from 38 percent in August to 48 percent in the latest WMUR poll) as well as winning them.

In Georgia, Perdue is painting Nunn (a nonprofit executive and daughter of former Democratic Sen. Sam Nunn) as too liberal for a generally conservative state.

But the races are close enough to give the underdogs hope of a last-minute surge.

Democrats have seized on a 2005 comment by Perdue that he had "spent most of my career" outsourcing factory jobs to other countries.

“Revealing himself as the nation's foremost advocate of corporations over people is creating positive progressive momentum against Perdue,” Josh Orton of the group Progressives United writes in a fundraising appeal. “We're this close to winning in Georgia.”

And in New Hampshire, Brown is honing the same message as are fellow GOP candidates in other states: that a vote for his rival is a vote to support the agenda of the current Democratic president. Shaheen “supports Obama’s failed foreign policy,” says one recent Brown campaign ad.