North Carolina politicians and pundits are gearing up for this year’s U.S. Senate election, which will be the top contest on the November ballot. Sean Trende of Real Clear Politics explains why the 2014 Senate contests hold such great importance nationally.

At President Obama’s current job approval rating, Democrats would likely suffer substantial losses in the upper chamber. On the other hand, if the president were to improve his standing, and if Republican candidates implode in some races, the GOP could break even or even lose a seat or two.

But aside from bragging rights, 2014 is a sideshow for the following two years. Obama is a lame duck, and the GOP already has blocking power with the House. A Republican majority in the Senate could provide added leverage by forcing Obama to use his veto, and it could force vulnerable Democrats to cast tough votes. But where it really matters — where it could make a huge difference — is in the next round of Senate elections.

Given the playing field that year, Republican losses are at the very least probable in 2016. In fact, they are likely. So if the GOP blows it this year, and if a Democrat wins the presidency in 2016, he or she could have a sizable — perhaps even filibuster-proof — majority. Moreover, if the GOP doesn’t win a decent-size majority this year, it stands a good chance of losing it in 2016 if Democrats have a good year.

Because 2010 was such a great GOP year, with Republicans winning seats in a deep-blue state like Illinois and in bluish-purple states like New Hampshire and Wisconsin, they will have an unusually high number of vulnerable seats in 2016. Similarly, Democratic vulnerabilities are limited to Nevada (where popular Gov. Brian Sandoval could challenge Harry Reid) and in Colorado. But Republicans failed to win those seats in 2010, and there’s hardly any guarantee that the playing field will be as favorable as it was that year, and candidates will be facing a presidential electorate to boot (which tends to be more favorable to the Democrats).

With the caveat that 2016 is a long way off, and that a lot could change, I identify the following seats where the GOP will have varying degrees of vulnerability: Florida, Illinois, New Hampshire, North Carolina, Ohio, Pennsylvania and Wisconsin. In addition, John McCain and Charles Grassley will be 80 and 83, respectively, on Election Day. If they were to retire, their seats could be quite vulnerable, depending on the national environment.