President Obama is greeted by Sen. Kay Hagan (D-N.C.) who has criticized the Obama administration on veterans issues, as he arrives at North Carolina Air National Guard Base in Charlotte, N.C., on Aug. 26, 2014. At right is Sen. Richard Burr (R-N.C.) (Charles Dharapak/AP)

The Senate map is Democrats' friend in the 2016 cycle. They are defending only 10 seats while Republicans have two dozen of their own seats to hold. But wait, it gets better. Seven of those 24 Republican seats are in states that President Obama won not once but twice: Florida, Illinois, Iowa, New Hampshire, Ohio, Pennsylvania and Wisconsin.

To win the majority back, Democrats need to win five of those seven seats in November 2016. (If Hillary Clinton, or some other Democrat, wins the White House in 2016, then Senate Democrats need to win only four of those seven. That's the exact path Republicans took to the Senate majority in 2014 when, needing a six-seat gain, they won all six of the states — Alaska, Arkansas, Louisiana, Montana, South Dakota and West Virginia — that Mitt Romney carried in 2012 and were represented by Democrats. (Republicans also won two states — Iowa and Colorado — that Obama carried twice and one, North Carolina, that Obama won in 2008 and Romney won in 2012.)

Of course, 2014 was a historically good year for Senate Republicans. The last time the party won more than nine seats in a midterm election was 1994 when they won 10.  Prior to 1994, you have to go all the way back to 1946 when Republicans netted 12 seats.


And, while the map looks great for Democrats on paper, several of those seven races look less rosy in reality. Iowa is a very tough Democratic pickup unless Sen. Chuck Grassley (R) decides to retire, which he insists he isn't going to do. Ohio Sen. Rob Portman is a gifted politician and fundraiser while the Democratic bench in the state is decidedly thin.  The Democratic fields in New Hampshire, Florida and Illinois are still quite muddled. And neither Sens. Pat Toomey (Pa.) nor Ron Johnson (Wis.) are political dead men walking. Not yet, at least.

There are also two genuinely vulnerable Democrats — Sens. Harry Reid (Nev.) and Michael Bennet (Colo.) — on the ballot in 2016.

Still, as the 2014 election revealed, the map and the math are huge factors in the battle for the Senate. Both are on Democrats' side this time around.

Below are the 10 most competitive Senate contests on the ballot in 2016.  The number-one-ranked race is the most likely to switch parties in 2016.

10. Kentucky (Republican-controlled): Sen. Rand Paul (R) is staffing up, as expected, for a presidential run next year, landing incoming Illinois Gov. Bruce Rauner's (R) campaign manager, Chip Englander, as his campaign manager and installing Doug Stafford as a top adviser, among other well-known GOP consultants. Paul still has to figure out how he's going to run for president and hold his Senate seat if that former race doesn't work out. His opponent in that effort could be none other than Alison Lundergan Grimes, given her role as Kentucky's chief elections officer. (Previous ranking: 10)

9. Ohio (R): Ohio is a swing state. But Portman isn't seen as particularly vulnerable in 2016. A lot of that is because he banked $5.8 million by the end of 2014. Another big reason is Democrats have a very slim bench in Ohio.  Among the names mentioned are Cincinnati Councilman P.G. Sittenfeld, former governor Ted Strickland, former congresswoman Betty Sutton and Rep. Tim Ryan. A Strickland comeback would be particularly interesting, but he passed on a rematch with Gov. John Kasich (R) in 2014. (Previous ranking: 8)

8. Florida (R): Whither Marco Rubio? The Florida senator could be the odd man out in the presidential race, with fellow Floridian Jeb Bush and other establishment-friendly candidates like Mitt Romney and Chris Christie in the mix. Consider this: Rubio is just 43 years old, and he's got his 2016 reelection campaign to worry about. Perhaps it's better to focus on staying in the Senate and waiting for the next opportunity. If he runs, he'll be favored. And Republicans would be happy to see him stay focused on the Senate. (Previous ranking: 9)

7. New Hampshire (R): The massive question here is what Gov. Maggie Hassan (D) does. Democrats think there's a pretty decent chance she'll run against Sen. Kelly Ayotte (R), but a New England College poll showed Ayotte leading that matchup by five points. Hassan, for what it's worth, would have to give up her job to run for the Senate; New Hampshire governors serve two-year terms. (Previous ranking: 7)

6. Colorado (Democratic controlled): No one though Bennet would even be in the Senate in 2016. After being appointed as a total unknown in 2009, Bennet ran a perfect campaign in 2010 and benefited from the fact that his Republican opponent was not very good. Given Colorado's swinginess, Bennet will be a major Republican target again. But who will Republicans put forward? Rep. Mike Coffman and newly elected state Attorney General Cynthia Coffman are mentioned — they're married — as is Rep. Scott Tipton. None of that trio has the political skills of newly elected Sen. Cory Gardner (R). )(Previous ranking: 5)

5. North Carolina (R): Sen. Richard Burr (R) isn't doing much to inspire confidence in Republicans about his future plans. His fundraising is weak — $720,000 on hand as of the end of September 2014 — and rumors continue to swirl about him not running again in 2016. (Burr has said he plans to run.)  If Democrats can convince former senator Kay Hagan (D) to run, this race moves up on the Line. But even if they don't, it's hard to see this not being a very competitive race in a presidential year. (Previous ranking: 6)

4. Pennsylvania (R): Attorney General Kathleen Kane (D) was once the Democrats' heir apparent opponent for Toomey. She now faces potential criminal charges for allegedly violating the secrecy of a grand jury. This isn't huge news, given these problems have been going on for a while now and her name wasn't really being floated for Senate anymore, but it does reinforce that Democrats have a recruiting question here. The establishment does not want 2010 nominee and former congressman Joe Sestak as its nominee, but Sestak is pretty gung-ho about running again. (Previous ranking: 4)

3. Illinois (R): State Attorney General Lisa Madigan is Democrats' first choice to take on Sen. Mark Kirk (R). But, she has passed up so many winnable races for higher office that it's hard to see why she runs this time. If not Madigan, then Democrats will turn to Rep. Tammy Duckworth or Rep. Bill Foster.  Kirk is an excellent fundraiser and knows the seriousness of the challenge he faces.  Much depends on the top of the ticket; if the Democratic presidential nominee wins Illinois by 15 points (Obama carried it by 17 points) then it's hard to see a path to victory for Kirk. (Previous ranking: 3)

2. Nevada (D): Reid got knocked down — hard — while working out at his home in Nevada. He got so banged up that he missed the first days of Congress and might not regain vision in his right eye. He said it won't affect his reelection decision. The bigger question is what Gov. Brian Sandoval (R) does. Does he wait for Reid to decide whether to seek reelection? Force his hand? This is a big question in a race Republicans would love to win to stem possible losses elsewhere. Meanwhile, former lieutenant governor Brian Krolicki, who blamed Reid for a 2008 indictment that Krolicki later beat, is considering running. (Previous ranking: 2)

1. Wisconsin (R): Johnson seems entirely unaware of the fact that he won a Democratic-leaning (at the federal level) state in 2010. He had the ninth most conservative record in the Senate, according to National Journal's 2013 vote ratings. Democrats are banking on former Sen. Russ Feingold, who lost to Johnson in 2010, running for his old seat.  If Feingold does run, he'll have to run a much better campaign that he did five years ago. If he doesn't run, expect Rep. Ron Kind, who has been itching to run statewide for as long as we can remember, to leap into the race. (Previous ranking: 1)