Senate Republican leader Mitch McConnell of Kentucky is smiling. For now. (AP Photo/J. Scott Applewhite)

Congratulations, Republicans!  You won the Senate majority! Now, can you hold onto it for more than two years?

Looking at the 2016 Senate map, there's reason for some doubt.  “I’m helping win the majority in 2014, and I’m making the point that 2016’s going to be a very different map for us,” Ohio Republican Sen. Rob Portman, who is up in 2016, told the New York Times earlier this year.

He's right.

Republicans will have to defend 24 seats as compared to just 10 for Democrats in 2016. And, the raw numbers don't even tell the whole story. Seven seats currently held by Republican incumbents -- Florida, Illinois, Iowa, New Hampshire, Ohio, Pennsylvania and Wisconsin -- were all carried by President Obama in 2008 and 2012.  And there is chatter about potential Republican retirements in Arizona and Iowa; if either John McCain or Chuck Grassley decided to call it a career, each of those races would be major Democratic targets.

On the other side of the coin, Republican takeover opportunities are few and far between. By far the most endangered Democrat is Senate Majority Leader Harry Reid who survived in 2010 but could be facing Nevada Gov. Brian Sandoval (R), who won a second term with more than 70 percent of the vote on Tuesday, in 2016. Reid has said he will run again although his demotion from Majority Leader to Minority Leader might make him rethink those plans. The only other Democrat who starts the 2016 cycle in serious jeopardy is freshman Colorado Sen. Michael Bennet, who, like Reid, was a surprise winner in 2010. Sen.-elect Cory Gardner's (R) convincing win over Sen. Mark Udall (D) on Tuesday in the Rocky Mountain State will undoubtedly energize Republicans although it's less clear what the GOP bench looks like for a race against Bennet.

Outside of those two seats, there's almost no vulnerability on the Democratic side. Even if Sen. Barbara Boxer (Calif.) or Barbara Mikulski (Md.) decide not to run again, both sit in very, very Democratic states -- particularly at the federal level.

To win back the Senate majority in two years time will, likely, require Democrats to net four (if they hold the White House in 2016) or five (if they don't) seats. Republicans currently control 52 Senate seats in the 114th Congress but Sen. Mark Begich (D) is behind by 8,000 votes in Alaska and likely to lose, and Sen. Mary Landrieu's chances don't look great in Louisiana's Dec. 6 runoff.

Five seats is certainly not out of the question -- although it might be a bit of a stretch -- given the Senate map of 2016. Of the 10 most vulnerable seats listed below, Republicans hold eight.

The number one ranked race is the most likely to flip party control in 2016. To the Line!

The GOP gained control of the Senate Tuesday night, taking hold of the legislative agenda in that chamber. Here are three of the policies Republicans are likely to tackle as they take the reins in January 2015. (Julie Percha/The Washington Post)

10. Kentucky (Republican-controlled): As Tuesday’s election showed, Kentucky isn’t exactly fertile ground for Democrats. But something interesting happened even as Mitch McConnell walloped Alison Lundergan Grimes: Democrats held on to their majority in the state House. That means Sen. Rand Paul (R-Ky.) can’t count on changing state law to be able to run for president and Senate at the same time. Hence: Possible open seat. First things first, though: the state's open governor's race in 2015, which will be the sole focus for the next year.

9. Florida (R): Sen. Marco Rubio (R) has suggested that he won't run for both president and reelection to the Senate. If he pursues the former and isn't on the Senate ballot, this becomes an open-seat race in a true swing state in a presidential year -- in other words, a good opportunity for Democrats. If Rubio passes on a White House bid or drops out with enough time to mount a Senate bid, Republicans would probably feel better about holding this seat. All that said, Democrats will have to find a strong recruit in either case, and no one jumps out right now.

8. Ohio (R): Portman is one of several Republican members of congress who have been mentioned (or mentioned themselves) as possible White House contenders. So, this could end up being an open seat. If Portman decides to run for reelection, his deep connections to donors through his work as National Republican Senatorial Committee vice chairman should ensure that he will be a financial behemoth. Portman is not terribly polarizing and there is no obvious Democratic recruit waiting in the wings. That's a recipe for success in this perennial swing state.

7. New Hampshire (R): The Granite State was one of the few bright spots for Democrats nationally as Sen. Jeanne Shaheen (D) beat back a challenge from Scott Brown. It could be a Senate battleground again in two years' time if Gov. Maggie Hassan (D) who won reelection with 53 percent on Tuesday, decides to take on freshman Sen. Kelly Ayotte (R).  There is also considerable chatter among conservative activists about a primary challenge to Ayotte although it remains to be seen whether a serious one might materialize. And, just to make things even more complicated, Ayotte is likely to be in the vice presidential mix no matter who wins the Republican presidential nomination.

6. North Carolina (R): The GOP picked off a seat here on Tuesday. It's safe to assume, however, that if the environment wasn't so good for the GOP, Kay Hagan would still be a senator come January. Her colleague, Sen. Richard Burr (R-N.C.) is up for reelection in 2016, and even if he doesn't retire -- he raised very little money the last two years, which is usually a precursor to retirement -- he will likely find himself targeted. Maybe even by Hagan (?).

5. Colorado (Democratic controlled): Sen. Michael Bennet probably doesn't want to think about 2016 just yet. He just finished a stint as chairman of the Democratic Senatorial Campaign Committee, in which he lost the Senate majority and became the first chairman in more than four decades to lose a home-state colleague in the process. Alas, it's time to ramp up. Bennet won by the narrowest of margins in 2010 and probably would have lost had Ken Buck, the Republican nominee, not said some unhelpful things. The question is whom the GOP recruits now that its sole real rising star is set to assume the state's other Senate seat.

4.  Pennsylvania (R): 2010 was about as good a year as a Republican could hope for in Pennsylvania.  And, Sen. Pat Toomey (R) still only won with 51 percent of the vote.  In a presidential year, Toomey's challenge will be even more serious; Republicans haven't carried the Keystone State at the presidential level since 1988. One thing working in Toomey's favor: A relatively weak Democratic bench.  State Attorney General Kathleen Kane apparently has no interest in running for the Senate. The only person actively looking at a bid is former Rep. Joe Sestak, who lost to Toomey in 2010.

3. Illinois (R): The first big question that needs to get answered in this race is whether Sen. Mark Kirk (R) runs again. Kirk, who suffered a severe stroke in early 2012, has insisted he plans to seek a second term but even some Republicans are taking a wait and see approach.  Democratic speculation -- no matter what Kirk does -- will center on state Attorney General Lisa Madigan but she seems a much more likely 2018 challenger to Gov.-elect Bruce Rauner (R). Assuming Madigan is a no-go, look for Rep. Tammy Duckworth (D) to be at the top of Democratic wish lists.

2. Nevada (D): Sen. Harry Reid will soon no longer be majority leader. The question is whether he wants to be minority leader and whether he sticks around. He's got bad personal approval numbers and is staring at a potential matchup with Sandoval. Tuesday's election was actually pretty big here. Not only did Sandoval cruise to reelection with 71 percent of the vote -- 71 percent! -- the GOP also cruised in the lieutenant governor's race,  a huge proxy war Reid badly wanted to win. That means Sandoval can run in 2016 without worrying about the governor's seat going to a Democrat.

1. Wisconsin (R): Sen. Ron Johnson starts the 2016 election cycle as the most vulnerable senator on the map. He's undefined in the eyes of many, polling shows; and he's running in a state that has gone Democratic in seven straight presidential elections. To boot, there are rumors that Democrat Russ Feingold, who Johnson unseated in 2010, may run. Feingold would start with high name recognition and a loyal liberal following.