Democrats are looking to reclaim their Senate majority in two years, but after losing nine Senate seats in 2014, their path back to that majority won't be easy.

Democrats will be benefiting from a favorable landscape, with Republicans defending 24 seats (many of them in blue territory) while Democrats will be defending only 10. To leverage that advantage into control of the Senate, however, Democrats need to net at least four seats (five, if Republicans win the presidency). That requires sweeping out blue-state freshman Republicans in states such as Illinois, Pennsylvania, and Wisconsin while also defeating a couple of brand-name senators, such as Rob Portman or Marco Rubio, in perennial swing states.

So to kick off the new year, here is National Journal's preview of the seven most compelling Senate races in 2016, with the most pressing question that will determine the race's outcome listed below:

Nevada: Does Governor Brian Sandoval run against Senate Minority Leader Harry Reid?

Despite sporting the worst approval ratings of any senator running for reelection in 2016, Reid is as well prepared as anyone to take on his competition—or at least to scare them away from running in the first place. The GOP's dream candidate, Governor Brian Sandoval, would probably start out ahead, thanks to his statewide political profile, moderate reputation, and Hispanic background. But he's far from a sure thing to run, knowing Reid's excellent track record in pulverizing his opposition. Giving up an influential job where he's become one of the GOP's nationally compelling politicians to wrestle in the mud with the Senate minority leader isn't exactly a no-brainer of a decision.

Reid, however, is looking vulnerable enough that Nevada political analyst Jon Ralston rates his chances as no better than 55 percent, no matter whom the Republicans run against him. But if Sandoval passes on the race, there's a greater chance that a hard-right candidate could emerge in a primary—and that's proven to be a surefire path to victory for Reid in the past. And the presidential-year electorate in 2016, with higher Hispanic turnout, should be very beneficial for Reid. That's one reason he was such an enthusiastic champion of President Obama's executive orders on immigration. It's no coincidence that Obama announced the decision, Reid by his side, in Las Vegas.

Wisconsin: If Russ Feingold runs, will he welcome the financial support of outside groups, including the DSCC?

During his time in the Senate, Feingold was the Democrats' leading campaign finance scold—and he lived up to his principles in practice. He refused any outside spending from the Democratic Senatorial Campaign Committee in his unsuccessful 2010 campaign, and from any other outside super PACs. But a lot has changed in the past four years, and without the assistance of outside groups, Feingold would be badly outspent. Senator Ron Johnson, who self-financed his way to a comfortable victory to win the seat, has the resources to do so again in two years.

Feingold has been coy about his future plans, but Democrats expect him to consider a comeback against Johnson. Feingold is a favorite of progressives, and his candidacy would be a reliable way to energize the grassroots base. But some Democrats would prefer a more-moderate nominee with a better relationship with party leaders, such as Representative Ron Kind.

Pennsylvania: Can Democrats handle Joe Sestak's independent streak, or will the party be divided against Pat Toomey?

Like Feingold, former Senate candidate Sestak isn't one to listen to the advice of party elders. Even though Democrats got Arlen Specter to switch parties in 2010, the retired three-star admiral challenged him anyway—and won, despite being attacked by his own party. And even though outside Democratic groups viewed his race as something of a lost cause, Sestak came within 2 points of defeating Republican Pat Toomey in a dismal year for Democrats across the country.

Since losing, Sestak has traveled across the state, all in preparation for his second run for the seat. But unlike many prospective Democratic candidates, he has offered some broadsides against his party leadership in the process. In September, Sestak bluntly told National Journal that Obama "failed in his handling of ISIS" and needed to assume more accountability. Sestak has been similarly critical of Reid, who aggressively backed Specter in the 2010 primary and didn't offer much financial support for Sestak in the general election. How Democrats will handle that straight talk—and whether Sestak faces a credible primary challenge—will go a long way in determining his chances of ousting Toomey.

Illinois: How will Senator Mark Kirk's health affect the race's competitiveness?

On paper, Kirk looks like the most vulnerable Republican senator. He's running for reelection in a deep-blue Democratic state, which Obama carried with 58 percent of the vote in 2012. Even in a historic year for Republicans, he defeated his Democratic challenger by only 2 points. The list of potential Democratic challengers is long, and it includes Representatives Tammy Duckworth, Bill Foster, and Cheri Bustos.

Kirk, who suffered a major stroke early in his Senate career, has been recovering well and has made it clear he's running for a second term. Both parties acknowledge that the illness changes the tenor of a highly competitive race. Republicans are worried that Kirk will struggle to run the type of hard-hitting campaign necessary to win in such a challenging environment. But Democrats also understand that they may have to pull some punches, running more of an issues-based campaign in a state known for its hardball politics.

Kirk has proven he can attract Democratic voters, regularly winning elections for a decade in a solidly Democratic North Shore House seat. And Governor-elect Bruce Rauner's victory in last year's governor's race, which was staffed by many longtime Kirk advisers, was seen as something of a test run for a reelection campaign.

North Carolina: Can Democrats recruit a credible candidate to run against Senator Richard Burr?

North Carolina should again be a competitive presidential battleground in 2016, but it's not clear whether Democrats have a deep enough bench to seriously challenge Senator Richard Burr. The party's strongest candidate in the state, state Attorney General Roy Cooper, is running for governor. And the party's congressional bench was decimated after the 2010 midterms and subsequent redistricting, leaving just three Democrats left out of 13 representatives. There's a reason Kay Hagan, despite losing a winnable race in 2014, is being mentioned as the best hope for contesting the seat.

Burr has thrived as one of the more anonymous senators, with one recent poll from the Democratic firm Public Policy Polling showing that one-third of North Carolina voters have no opinion about him. Yet he's won election twice despite his low profile. That could remain the case in 2014, unless Democrats recruit a star candidate.

Florida: Will Marco Rubio be running for a second term?

Jeb Bush's decision to run for president complicates Rubio's own interests for higher office. With many of the freshman senator's most reliable donors lining up with Bush, it's increasingly possible that Rubio will opt to run for reelection. But what's bad for Rubio's national ambitions would be good for the GOP's chances of holding onto the swing Senate seat.

If Rubio chose to stick in the Senate, it could complicate the math for Democrats. Would second-term Representative Patrick Murphy or newly elected Representative Gwen Graham want to risk their developing House careers for a shot at the Senate? Would Democratic National Committee Chair Debbie Wasserman Schultz consider a run? All of their odds of running would be higher if this was an open seat. And Rubio's high-profile advocacy against the president's deal normalizing relations with Cuba should help him rally the Cuban-American GOP base in a reelection contest.

Arizona: Does John McCain get a primary challenger?

With immigration reemerging as a leading issue, McCain could face another credible primary challenge in 2016, if he chooses to run for a sixth term. In the run-up to his last campaign, he moved to the right on immigration, retreating from supporting a path to citizenship and supporting the completion of the border barrier between the U.S. and Mexico. His changed positions helped him coast to victory in the primary against former GOP Representative J.D. Hayworth. But after being reelected, he again led Senate efforts for comprehensive immigration reform, only to see it stalled once again.

It's unclear whether immigration will be as hot an issue in 2016 as it was in the run-up to the 2014 midterms. But Republicans have a deep bench of conservatives who could consider a primary challenge, including Representative Matt Salmon and David Schweikert. And if McCain looks beatable, outside conservative groups—which are upset with McCain on issues other than immigration—might try to focus all their intra-party efforts against the senator for 2016. If McCain loses the primary, it could put the seat in play for Democrats.