Of the 100 Senate seats, 36 seats are up for election, 21 of them Democratic and 15 Republican.
Eight of them -- Alaska, Arkansas, Colorado, Georgia, Iowa, Kentucky, Louisiana and North Carolina -- are tossups. According to Real Clear Politics, Georgia and Iowa are open seats, i.e., no incumbent is running, and they were previously held by a Republican and a Democrat respectively.
The other states, with the exception of Kentucky, where the seat is held by Republican Minority leader Mitch McConnell, are held by Democrats. With the Senate currently split 53 Democratic, 45 Republican and two independents, a shift to a majority Republican Senate became more probable after this week's primary.
What is at stake is less about local or state politics and more about the national stage.
Three primaries held this week shed light on the national mood. In Mississippi, six-term Republican Sen. Thad Cochran ran against State Sen. Chris McDaniel, who was championed by the tea party.
McDaniel had received 1,418 more votes than Cochran in the primary and was attempting to vilify the incumbent as an establishment insider. In the past few days, a number of national figures have joined the fray, endorsing one or the other candidate: Former Sen. Rick Santorum campaigned for McDaniel and Sen. John McCain campaigned for Cochran. While McDaniel focused on the right, Cochran focused on the middle -- independents and crossover Democrats.
Cochran won the runoff 51 percent versus 49 percent. This kept the state in the likely GOP column. A win by McDaniel would have left space for the Democratic nominee, former Rep. Travis Childers, to appeal to the middle and potentially capture the seat.
In Oklahoma, two candidates were competing in the primary to replace Sen. Tom Coburn; a Republican who is retiring: Rep. James Lankford; and former state House Speaker T.W. Shannon.
Shannon, part Native American and part African-American, was backed by the tea party and championed by Ted Cruz, the junior senator from Texas.
Lankford, who has served two terms in the House, rapidly rose in the House leadership and is the fifth-ranking House Republican. While this provided an opportunity for Shannon to cry "Establishment!" and "Insider!," Langford's background as a Baptist minister, his ability to inspire and call people to action gave him the edge.
As Nathan Gonzalez, deputy editor of the nonpartisan Rothenberg Political Report, told CNN, "The Oklahoma primary doesn't fit neatly into the establishment versus anti-establishment box. It's very different than some of the other high-profile races, where you had a challenger taking on an incumbent. The political insider in Washington may not look like the political insider in Oklahoma."
Lankford won the primary by a huge margin -- 57 percent to 34 percent. While this race would not have changed national results at the Senate level, it reminds us that it's about turnout and votes in the end.
In Colorado, former Rep. Tom Tancredo was running for the Republican nomination for governor in a field of four. They were all vying for the right to run against Gov. John Hickenlooper.
What is fascinating about this race is that there was less concern about who would run against Hickenlooper than there was about the potential impact on the Senate race in Colorado, where the Republican nominee, Cory Gardner, has pulled within two points of Democratic incumbent Mark Udall.
Left-leaning groups were running advertisements in support of Tancredo in the apparent hope that he would win the primary and that his hard stance on immigration would serve as a lightning rod to attract Democratic voters to the polls in November to vote against him and, at the same time, for Udall.
The win over Tancredo by Bob Beauprez means Republicans are more likely to gain a Senate seat this fall in Colorado.
This year, it's all Senate -- all the time.