We are now four full months into the 2016 election cycle, and the battle for the Senate is starting to take shape.

Retirements in California, Florida, Indiana, Maryland and Nevada have set up open-seat contests -- though only two of them are considered top-tier races. And with Sens. John McCain (R-Ariz.) and Chuck Grassley (R-Iowa) gearing up for reelection and saving Republicans major headaches in the process, we have a pretty good idea of where the battleground races will be.

The biggest question from here is recruiting. Republicans have big holes to fill in the two competitive open seats, in Florida and Nevada, where, despite weeks-old retirements, no major candidates have jumped in. The same goes in Colorado, a top pickup opportunity against Sen. Michael Bennet (D-Colo.) where the field has yet to really take shape.

Democrats have an early frontrunner in both of those open seats, though it's far too early to call them favorites in these swing states. At the same time, they still have a sizable recruiting hole in Pennsylvania and are likely to face a tough primary even if they land a candidate not named Joe Sestak. And in Illinois, Rep. Tammy Duckworth (D) is in and appeared to be clearing the field, but Rep. Robin Kelly is now looking at running too.

Democrats are also waiting to see if former senators Russ Feingold (D-Wis.) and Kay Hagan (D-N.C.) will run, and if popular New Hampshire Gov. Maggie Hassan (D) gives them a shot against Sen. Kelly Ayotte (R).

In other words, we are close to knowing the states that will decide control of the Senate in 2016, but the candidates in many of these contests are still big question marks.

As always, the races are ranked from most likely to change hands at No. 1 to less-likely-but-still-in-play at No. 10.

10. Indiana (Republican-controlled): Democrats appear to have new life in the Indiana governor’s race, with Gov. Mike Pence (R) running into trouble and former state House speaker John Gregg (D) opting for a rematch. That’s probably good news for Democrats in the open Senate race, but it’s still an uphill battle. The lone major candidate here is former state GOP chairman and congressional aide Eric Holcomb. (Previous ranking: 10)

9. North Carolina (R): A poll this week from Elon University shows Hagan (D) within one point of Sen. Richard Burr (R). But we’re still waiting for Hagan, who lost her seat in November, to make her intentions known, and unlike some other potential repeat candidates below, it’s less clear it will happen. (Previous ranking: 9)

8. Ohio (R): Eyebrows were raised when a Quinnipiac University poll showed former Gov. Ted Strickland (D) leading Sen. Rob Portman (R) 48 percent to 39 percent. Republicans didn't strongly dispute the numbers but insisted that the poll represented Strickland's high water mark because of a lingering name ID edge.  Portman is extremely able as a candidate and fundraiser and he and his campaign team are well aware of the challenge before him. (Previous ranking: 8)

7. New Hampshire (R): The waiting, as Tom Petty knew, is the hardest part. Nothing really moves in this race until Hassan decides whether or not she wants to take on Ayotte.  That decision isn't likely until June (at the earliest). In the meantime, Ayotte looks to have dodged a serious primary challenge in the form of Ovide Lamontagne, a conservative who nearly upset her in 2010. That's good news for Republicans. (Previous ranking: 7)

6. Colorado (Democratic controlled): Republicans pulled a rabbit out of a hat in Colorado in 2014 when they convinced then Rep. Cory Gardner (R) to run for Senate after he initially refused.  Can they do it again? No one thinks Bennet is safe but the Republican field is, literally, nonexistent at the moment. Republicans will find someone but there doesn't appear to be anyone of Gardner's ability waiting in the wings. (Previous ranking: 5)

5. Pennsylvania (R): Former congressman Joe Sestak is still the Democratic frontrunner to face Sen. Pat Toomey (R). Allentown Mayor Ed Pawlowski just got into the primary, too, but his 2014 governor campaign flamed out early. National Democrats, meanwhile, appear to be leaning on Montgomery County Commissioner Josh Shapiro to get in the race and avert another nomination of Sestak, who lost to Toomey in 2010 and rubs party leaders the wrong way. (Previous ranking: 4)

4. Nevada (D): Republicans are still in search of their standard-bearer here, more than a month after Senate Minority Leader Harry Reid (D) announced he would retire. On the Democratic side, former state attorney general Catherine Cortez Masto is running and has Reid’s backing, but Rep. Dina Titus is still looking at her own bid. Potential GOP candidates include Rep. Joe Heck, who is exploring a bid, as well as Lt. Gov. Mark Hutchinson, former lieutenant governor Brian Krolicki and state Senate Majority Leader Michael Roberson (R). (Previous ranking: 2)

3. Florida (R): Speaking of places where the GOP field is in flux (or rather, doesn’t yet exist), a series of Republicans have passed on running since Sen. Marco Rubio (R) announced his presidential campaign a few weeks ago. Among them: state Chief Financial Officer Jeff Atwater, state House Speaker Will Weatherford, Rep. Tom Rooney and Rep. Vern Buchanan. The remaining potential candidates include Reps. Ron DeSantis and David Jolly, Lt. Gov. Carlos López-Cantera and state Sen. Don Gaetz. Rep. Patrick Murphy is running on the Democratic side, but could face a primary against liberal firebrand Rep. Alan Grayson. (Previous ranking: 6)

2. Wisconsin (R): Assuming Feingold runs -- and we assume he does -- Sen. Ron Johnson (R) is in deep trouble. The only two polls conducted in this race show Feingold with leads of nine and 16 points, which is a very tough place for any incumbent to start.  Republicans' best hope may be that Feingold ran an atrocious campaign against Johnson  -- when the Democrat was the incumbent -- in 2010, and maybe he will do the same again this time. (Previous ranking: 3)

1. Illinois (R): Duckworth is the preferred candidate of national Democrats -- and has won the endorsement of EMILY's List. But Kelly, who won a special election for the seat of former Rep. Jesse Jackson Jr. in 2013, could pose some problems for Duckworth in a primary because of her base in the city of Chicago.  Kirk is deeply endangered, having barely won the seat in a midterm election against a deeply flawed Democratic challenger. (Previous ranking: 1)