The ten closest Senate races in the U.S.

It’s a nail-biter.

And we’re not just talking about the presidential race.

Thirteen of the 33 Senate contests remain up for grabs with just a week left before the election. And there’s a lot at stake.

If the current leader ends up winning, the Republicans will net one seat in the Senate, leaving the Democrats with a narrow majority. The best case scenario for Republicans is a gain of nine seats and a solid majority. The Democrats’ best case scenario is a pick-up of four.

Here are the ten closest Senate races in the country, based on our analysis of RealClearPolitics data. And thanks to Max Kranl of the Hearst Washington bureau for collecting the numbers for us to slice and dice:

1. Montana: Denny Rehberg (R) vs. Sen. Jon Tester (D)

Average: Rehberg +0.3
Range: Rehberg +3 to Tester +2
Most recent polls: Rehberg leads one poll, Tester leads one, one a tie
Polling reliability: High
Rick Dunham’s analysis: Can Democrats turn out their voters in a red state to save their most endangered incumbent?

2. Arizona: Jeff Flake (R) vs. Richard Carmona (D)

Average: Flake +0.7
Range: Flake +6 to Carmona +4
Most recent polls: Flake leads in two polls, Carmona also leads in two
Polling reliability: Low
Rick Dunham’s analysis: Wild fluctuations in polls in an unexpectedly competitive race for a GOP-held seat.

3. Wisconsin: Tammy Baldwin (D) vs. Tommy Thompson (R)

Average: Baldwin +0.8
Range: Baldwin +4 to Thompson +2
Most recent polls: Baldwin and Thompson each lead in two polls
Polling reliability: High
Rick Dunham’s analysis: Thompson running just ahead of Romney-Ryan ticket in a key presidential battleground.

4. Virginia: Tim Kaine (D) vs. George Allen (R)

Average: Kaine +1.0
Range: Kaine +7 to Allen +5
Most recent polls: Kaine leads in three of four polls, Allen in one
Polling reliability: Moderate
Rick Dunham’s analysis: Kaine running just ahead of President Obama in dead-heat state.

5. Indiana: Richard Mourdock (R) vs. Joe Donnelly (D)

Average: Mourdock +1.3
Range: Mourdock +5 to Donnelly +2
Most recent polls: Mourdock leads in two polls, Donnelly in one, one a tie
Polling reliability: High
Rick Dunham’s analysis: Highly volatile race after Mourdock’s “rape” comment created a firestorm.

6. Connecticut: Chris Murphy (D) vs. Linda McMahon (R)

Average: Murphy +3.1
Range: Murphy +6 to tie
Most recent polls: Murphy leading in five of six polls, one a tie
Polling reliability: High
Rick Dunham’s analysis: Murphy has rebounded from September slump but hasn’t put race away.

7. Nevada: Sen. Dean Heller (R) vs. Shelley Berkley (D)

Sen. Scott Brown on Capitol Hill (AP photo)

Average: Heller +3.5
Range: Heller +6 to tie
Most recent polls: Heller leading in three of four polls, one a tie
Polling reliability: High
Rick Dunham’s analysis: Romney rebound has helped Heller.

8. Massachusetts: Elizabeth Warren (D) vs. Sen. Scott Brown (R)

Average: Warren +4.7
Range: Warren +6 to Warren +2
Most recent polls: Warren leading in all three polls
Polling reliability: High
Rick Dunham’s analysis: The trendline favors Warren.

9. North Dakota: Rick Berg (R) vs. Heidi Heitkamp (D)

Average: Berg +5.0
Range: Berg +10 to tie
Most recent polls: Berg leading in two of three polls, one a tie
Polling reliability: Low
Rick Dunham’s analysis: Romney romp may rescue Berg from strong foe.

10. Missouri: Sen. Claire McCaskill (D) vs. Todd Akin (R)

Average: McCaskill +5.0
Range: McCaskill +6 to McCaskill +1
Most recent polls: McCaskill leads in all three polls
Polling reliability: High
Rick Dunham’s analysis: McCaskill hasn’t been able to close the deal against a weak opponent.

Other races to watch: Pennsylvania, Ohio and Florida