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One of the most powerful threats Senate Republicans are using in their push to block Senator Harry Reid's call for reform of the filibuster is a simple one: We could be in the majority in 2014. According to race-by-race analysis from 538's Nate Silver, that's a real risk for Senate Democrats.

As our colleagues at National Journal pointed out on Monday, Democrats' hopes of keeping the Senate after the 2014 elections were dealt a blow with the announcement that Montana Gov. Brian Schweitzer wouldn't run for the seat. In Silver's estimation, that makes Republicans "three-to-one favorites based on the state's overall partisan lean." That's similar to two other states with Democratic senators that are retiring, West Virginia and South Dakota. That same calculus only works in the Democrats' favor in one state, New Jersey, where the party will almost certainly regain the seat lost with the death of Frank Lautenberg.

But as we've seen lately, anything can happen in senatorial politics. We've created a series of maps showing how the Republican caucus stands to grow in 2014, based on Silver's estimates. (Which you should review if you're interested in this stuff; he does a state-by-state breakdown.) Each of the first two maps includes both senators from the state. The darker the red, the more likely both senators will be (or are) Republican. Numbers are percentages: Sitting or uncontested Republicans get 100 percent, contested races are tallied at the percentage likelihood of Republican victory. Both states senators are then added.

Switch maps: Current Senate makeup | Silver's 2014 likelihood | Net change likelihood

Current Senate

As you can see, the change is fairly subtle, largely comporting with Silver's analysis. The two question marks he isolates are Louisiana and Arkansas, where Democratic incumbents — Landrieu and Pryor — face uphill battles to retain their seats. As shown in the graph below, those two seats make all of the difference. If the Republicans win both, the Senate drops to a 50-50 split, with Vice President Biden being called in for ties a lot more often. If both incumbents win, the Democrats and their independent allies retain control of the body.

There are a lot of question marks. The National Republican Senatorial Committee has had a run of bad luck with the nominations of untenable (Christine O'Donnell), gaffe-prone (Todd Akin), and inept (Sharron Angle) candidates. Republicans have been expected to regain the Senate before, without it happening. For 2014 to be their year, they need luck to be on their side for a change.

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