We can predict the results of most of those races well in advance — as we did in July. Twenty-one Democratic seats are up, as are 14 Republican seats. Seventeen of those races, as Nate Silver predicted over the summer, will almost certainly be won by Republicans, a net gain of three seats right off the bat. It's conceivable — unlikely, but conceivable — that the Republicans could do much better than that, picking up a filibuster-proof majority.
On Sunday, Politico's James Hohmann outlined a number of the races, suggesting that a dramatic shift is unlikely, and pointing out that this is the third-straight cycle that the Republicans might have taken the majority. Each of the past two times, the party has been stymied by the nominations of far-right candidates that went on to lose the general election: Sharron Angle, Richard Mourdock, Christine O'Donnell, and so on. The Senate Republican campaign committee is promising to try and weed out unelectable candidates early, but it's not clear they will be able to do so.
Each possible post-2014 outcome means different things for the 2015/2016 Senate — but also means different strategies for the Senate right now.
Scenario 1: The Democrats retain the majority.
Likelihood: high. Obama's nominees to sit on the D.C. Circuit Court of Appeals, the appeals court responsible for weighing in on a broad swath of federal legislation, have consistently been blocked by the Senate Republicans, using their ability to filibuster to force the need for a 60-vote supermajority before nominees can move forward. Last week, Massachusetts Sen. Elizabeth Warren lambasted Republicans, insisting that they were trying to nullify the 2012 election by not allowing the president to nominate his choices to the court.
The Senate Democrats have repeatedly threatened to revoke the right of the minority to filibuster appointees — but haven't done it, in part because of that nagging question of what happens when the power in the chamber flips. In its assessment of the likelihood of a rules change, Politico notes that this concern may be stronger with a "new generation of senators" — those who will serve in the Senate longer and are more likely to see the power balance flip.
If it becomes clear over the short term that the Democrats will retain control of the Senate, that the Republicans won't get the six vote margin they need to assume control of the chamber,Democratic leaders may feel more empowered to make the rules change. The Democrats would need to lose both the presidency and the Senate after 2016 in order for this to become a problem for the party — and even then the D.C. Circuit would need to see additional retirements in order for that Republican president to get the ability to make appointments.
Scenario 2: Republicans get a filibuster-proof majority.
Likelihood: Very low. If 2014 turns into a Republican landslide election, sweeping 25 new Republican senators into office, the significance goes far beyond procedural issues in the Senate. It would mean wins in races that Republicans currently trail by wide margins, meaning that the race itself is likely a repudiation of Democrats and the President. If this were to happen, Obamacare would certainly be at risk — as would a host of other legislation.