Brian Darling
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· In the race to replace Sen. Lugar (R-Ind.), conservative Richard Mourdock is battling with three term Democratic Rep. Joe Donnely. Romney is expected to win this state by over 10 percentage points, yet polling shows the Senate race close, with Mourdock support softening in the last week.

· In Massachusetts, incumbent Republican Scott Brown’s trails the Democratic challenger, Elizabeth Warren. Brown won his Senate seat in a special election. This time he’s running in a general election, with President Obama at the top of the ticket. That makes for tough going in this Democratic stronghold.

· In Nevada, incumbent Sen. Dean Heller is struggling to fend off Democratic Rep. Shelley Berkley.

Meanwhile, the Democrats are playing defense on seven nail biters:

· Montana—A new Rasmussen poll has Sen. Jon Tester (D) and Rep. Denny Rehberg (R) within the margin of error. Romney is expected to win Montana big.

· North Dakota—Rep. Rick Berg (R) seems to be gaining steam, with a very good chance of defeating Heidi Heitekamp in the race to replace Democrat Kent Conrad.

· Connecticut—Republican Linda McMahon trails Rep. Chris Murphy (D) in the contest to replace retiring Independent Joe Lieberman.

· Wisconsin—Moderate Republican Tommy Thompson, a former governor, faces liberal Democratic Rep. Tammy Baldwin for Sen. Herb Kohl’s seat. The state is divided evenly between the parties, yet has gone Republican in two recent statewide elections—Governor Scott Walker and U.S. Senator Ron Johnson

· Virginia—A presidential swing state, the Senate race here features former Republican Sen. George Allen against former Democratic Governor Tim Kaine.

· Ohio—Sen. Sherrod Brown (D-Ohio) retains a polling lead against Josh Mandel.

· Missouri—Sen. Claire McCaskill leads Rep. Todd Akin. In the wake of controversial remarks about rape and conception, Akin was abandoned by national Republican leaders in a race that may determine whether Sen. Mitch McConnell (R-Ky.) or Sen. Harry Reid (D-NV) holds the gavel in the next Congress.

My take? A big night for either Obama or Romney will dramatically impact the 11 Senate toss-ups. If Romney romps, most of the seven Democratic-held toss-up seats could flip. (Montana and North Dakota may flip, even with an Obama victory.) Ohio, Missouri and Connecticut seem to be the most difficult contests for Republicans. McMahon will have a tough time, even with a Romney triumph. The bottom line: A good night for Republicans would get them to 51 seats.

Best case scenario for Democrats hinges on a big win for Obama. Under that scenario, they hold all their endangered seats (with Montana still being difficult) and pick up maybe two of the GOP-held cliff hangers. Hence, a good night for Democrats would net a one seat pick-up, giving them 54 seats.

Election night isn’t just about who gets to sit in the Oval Office. Who rules the Senate will go a long way toward determining which policies ripen and which wither on the vine throughout the next session of Congress.

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Brian Darling

Brian Darling is a Senior Fellow in Government Studies at the Heritage Foundation. Follow him on Twitter @BrianHDarling