If the Republicans gain control of the Senate tomorrow, an outcome some observers are predicting but which is by no means assured, there will be a number of consequences, two of which are these:
It will signal an end to congressional gridlock. Many Americans complain that Congress can't get anything done and they're correct to think that. However, part of the reason is that when the House of Representatives, which is controlled by the Republicans, passes legislation it must go to the Senate, currently controlled by the Democrats, to be voted on before it becomes law. Currently, there are some 340 pieces of legislation passed by the House which the Democrats in the Senate, whether committee chairs or the Majority Leader, refuse, for various reasons, to bring to the floor for a vote.
If the GOP holds the House (which is pretty much a certainty) and wins a net six Senate seats they'll effectively control both chambers and thus legislation will move much more smoothly through Congress. That's not to say it'll make it into law because it'll still have to be signed by the president, who may not be disposed to sign Republican initiatives, but at least the bottleneck won't be Congress.
The second consequence is that the president will be forced to temper his predilection to nominate appointees who reside on the far left of the political spectrum. Such nominees will not likely be confirmed by a Republican Senate. This will be especially significant for nominees to the federal bench, and a forteriori, to the Supreme Court, where there could be at least two retirements in the next two years.
One fear that many Democrats have about a GOP takeover of the Senate seems to me to be unfounded. The concern is that if the Republicans control both houses they'll seek to impeach President Obama. I doubt this very much unless Mr. Obama does something extraordinarily reckless. The way impeachment works is that the House impeaches the office-holder and then the case goes to the Senate where the impeached individual is essentially put on trial.
In order to convict and have the person removed from office it takes the vote of two-thirds of the Senate. That'd be 67 senators. Even if every Republican voted for it (something that didn't happen when President Clinton was impeached), there would still have to be some fifteen or sixteen Democrats who'd have to join them. The chances of finding this many Democrats who would vote to remove a president of their own party are slimmer than zero.
The Republicans wouldn't waste the resources on such an effort unless they thought it had a chance of success, and it would have such a chance only if the American people and the media were howling for impeachment, a prerequisite that, as of now, seems as remote as the planet Pluto.