Republicans took another step toward taking back the U.S. Senate when Sen. Tom Harkin (D-Iowa) announced this weekend that he would not run for re-election in 2014.

While Harkin likely faced a close race, he was the candidate most likely to hold the seat for Democrats. Now his party faces a real prospect of losing the seat. The Rothenberg Political Report notes that without Harkin on the ballot, "the Hawkeye State now moves far up on GOP target lists."

Combined with Sen. Jay Rockefeller's (D-W.V.) retirement announcement earlier this month, it's a real blow to Democrats.

Here's what's at stake in the midterm elections: Democrats will defend 21 Senate seats while Republicans need to defend just 14.

But as the Washington Post notes, it's an even steeper climb than it looks. Seven of the seats Democrats will defend are in states that voted Republican in the 2012 presidential election: West Virginia, Arkansas, South Dakota, Louisiana, Alaska, Montana, and North Carolina. And in six of those seven — all but North Carolina — Obama lost by double-digits.

Interestingly, with Democrats currently holding a 55-45 majority in the Senate, Republicans must flip 6 seats to take back control. Those six states the GOP won handily in 2012 are their path back to power.

For now, the GOP-controlled House of Representatives is President Obama's biggest obstacle to his agenda on Capitol Hill. But come 2014, he may face roadblocks in both houses of Congress.