All eyes on the Senate

The sun begins to set behind the US Capitol in Washington, D.C.
The sun begins to set behind the US Capitol in Washington, D.C.
As Election Day gets underway, there's no shortage of important races to watch, but under the circumstances, it's clear that control of the U.S. Senate is getting the bulk of the attention -- major, national institutions do not change hands often, and Republicans are well positioned to take control of the chamber after eight years in the minority.
But it's probably not best not to think of this in an entirely binary way, with one party having a good night or the other. There are degrees to keep in mind.
Scenario #1: Republicans meet their goal
On average, as Rachel noted on the show last night, the president's party loses six Senate seats in a sixth-year midterm. In order for the GOP to seize the Senate, they'll need a net gain of ... six seats.
How likely is this scenario? It's a pretty safe bet, actually. FiveThirtyEight's final analysis found there's a 73% likelihood that Republicans will take the majority. The Upshot's forecast put the number at 68%. The Huffington Post's analysis gives the GOP a whopping 79% chance. Remember when the Princeton Election Consortium was the only one with a forecast that favored Democrats? Not anymore -- Sam Wang & Co. now show Republicans with a 63% edge.
With Democratic incumbents retiring in West Virginia, South Dakota, and Montana, Republicans are effectively halfway to their goal already. From there, if the GOP gets a net gain of three from the remaining competitive races, the party will have met its goal.
Scenario #2: Democrats narrowly hang on to their majority
The odds are quite obviously against Democrats holding onto their majority for the fifth consecutive Congress, and the smart money is on GOP control, but the Dems' path is not too outlandish.
If, for example, Hagan holds on in North Carolina (she's favored), Nunn wins a runoff in Georgia (she looked like a decent bet as recently as a week ago), Shaheen hangs on in New Hampshire (she's favored), Begich prevails in Alaska (he's led in some recent polls), the Democrats' ground-game in Colorado is as good as advertised (the polls were wrong in 2010), and Orman prevails in Kansas (a distinct possibility), they keep their majority with 51 seats. If one of these races goes the other way, a 50-50 Senate emerges -- with Vice President Biden becoming the tie-breaking vote.
Throw in a surprise or two in Iowa and/or Louisiana, and Dems will come out of this cycle with a sigh of relief that will echo from coast to coast.
Again, there's ample reason to be skeptical about this, but this scenario is plausible.
Scenario #3: Republican landslide
Democrats are right to worry about Republicans getting a net gain of six seats, at which point Mitch McConnell gets a nicer office. Dems might also consider, however, worrying about a far worse scenario: the very real likelihood that Republicans are going to run the table in 2014, at least in Senate races.
Under this scenario, the question is whether Republicans will end up with 52 to 54 seats, not whether they'll end up with a simple majority.
This remains a real possibility. Republicans are favored to keep their seats in Kentucky and Georgia. At the same time, there are polls showing a GOP edge in Arkansas, Louisiana, Alaska, Iowa, and Colorado. North Carolina and New Hampshire are arguably in play.
If Republicans win each of them, that's a net gain of 10 seats.