The November election is 135 days away, but we now have a very clear idea of what the Senate playing field will look like.

Primaries have largely sorted themselves out in the most competitive Senate races in the country, with Republicans — so far — avoiding the perils of 2010 and 2012, in which the party nominated several candidates who had major electability problems in the general election.

What we are left with is 12 races that can be considered truly competitive — meaning that either one (or both) of the national parties and/or the various outside groups have spent or will spend money on them. The races are tipped heavily toward Democratic-held seats; 10 of the 12 in contention — including the six most vulnerable — are in Democratic hands. Of the 12 states, Republican Mitt Romney carried nine in 2012.

Republicans insist that the playing field is actually 14, not 12 — adding Minnesota and Oregon to the list. We remain unconvinced that Republican challengers in either of those Democratic-leaning seats have shown the ability to make the races genuinely competitive just yet. Similarly, Democratic optimism in Mississippi seems excessive to us — even if state Sen. Chris McDaniel ousts Sen. Thad Cochran in the GOP runoff Tuesday.

Below we’ve ranked the 12 most competitive Senate races in the country. The No. 1 race is the most likely to switch party control.

Election Lab: See our current forecast for every congressional race in 2014

12. Michigan (Democratic-controlled): Republican Terri Lynn Land was not the first — or even second — choice of many Republican strategists. But she has raised money at an impressive pace and has kept this race against Rep. Gary Peters close. The question for Land is whether she can sustain it when media and voter attention ramps up in the fall.

11. Georgia (Republican-controlled): We’re still awaiting the results of the Republican primary runoff on July 22. Businessman David Perdue beat Rep. Jack Kingston 31 percent to 26 percent on primary day, but there are lots of voters up for grabs. We’re not sure which man gives the GOP a better shot against Democrat Michelle Nunn — we’d lean toward Perdue — but Nunn is still polling well and raising big money.

10. Iowa (D): Republicans got a big break when state Sen. Joni Ernst routed the competition in the GOP Senate primary, and since then several polls have shown the race with Rep. Bruce Braley (D) to be something close to a toss-up.

9. Colorado (D): Although most people look at the North Carolina race as the one on which Senate control might swing, the race between Sen. Mark Udall (D) and Rep. Cory Gardner (R) could easily fit that bill, too. Polling by Quinnipiac in late April showed the race as a dead heat, and both sides acknowledge that the race is and will stay close. Udall is on TV now bashing Gardner as too conservative — particularly on abortion — and Democrats think Gardner’s record is full of bad votes.

8. Alaska (D): Conservative blogger Erick Erickson this week endorsed Lt. Gov. Mead Treadwell (R), giving the long-struggling candidate a much-needed boost. But it’s not going to change the fact that former attorney general Dan Sullivan is the clear front-runner for the Republican nomination. Sullivan has both establishment (American Crossroads) and tea party (Club for Growth) money in his corner and is well on his way to a fall showdown against Sen. Mark Begich (D).

7. Kentucky (R): Republicans are feeling more confident about Sen. Mitch McConnell’s chances after the Republican leader’s convincing primary victory last month and their sense that the GOP is quickly uniting behind him. And President Obama didn’t do Democrat Alison Lundergan Grimes any favors with his announcement on power plants this month.

6. Arkansas (D): Republican Rep. Tom Cotton’s campaign released an internal poll claiming a lead over Sen. Mark Pryor (D). The release was meant to counteract a growing narrative that Pryor is not nearly as vulnerable as he once seemed. But the most notable part of the survey was in the trend line: Cotton was polling the race in February 2013, when he was still a brand-new member of the House.

5. North Carolina (D): Democrats tried to make an issue of state House Speaker Thom Tillis (R) referring to white people as the “traditional population” of North Carolina. We wouldn’t call that a campaign-stopping gaffe, but given that Democrats would love to motivate minority voters in a midterm election, Tillis should probably choose his words a little more carefully. The race between Tillis and Sen. Kay Hagan (D) remains very close.

4. Louisiana (D): Expect Sen. Mary Landrieu (D) to emphasize (and reemphasize) her role as head of the Senate Energy Committee for the rest of the campaign. The question is whether the emphasis on energy will be enough to overcome a capable Republican opponent in Rep. Bill Cassidy in a state where the president is deeply unpopular.

3. Montana (D): Both appointed Sen. John Walsh (D) and Rep. Steve Daines (R) easily dispatched nominal primary challenges on June 3 and formally began a race that both campaigns had already been waging for months. There’s very little public polling in the race, but the general consensus is that Daines starts the general election with an edge.

2. West Virginia (D): Rep. Shelley Moore Capito (R) probably wrapped up this Senate seat in November 2012 when she abruptly announced her run even though Sen. Jay Rockefeller (D) had yet to announce his retirement. Democrats eventually persuaded Secretary of State Natalie Tennant to run, but she started the race at a distinct disadvantage because of Capito’s early start and the dislike toward the national Democratic Party in the state.

1. South Dakota (D): The Jackrabbit State remains our most likely seat to flip. But let’s make the case for this being in play. Former governor Mike Rounds (R) remains a strong favorite against Democrat Rick Weiland, but this race also includes former three-term GOP (U.S.) senator Larry Pressler and former Republican state senator Gordon Howie (not to be confused with Gordie Howe) running as independents. Neither has raised any money, but maybe Pressler (and, to a lesser extent, Howie) steal enough of Rounds’s votes that this is in play. Weiland can hope.

Aaron Blake and Sean Sullivan contributed to this report.