Barring some sort of systematic polling-collapse/miraculous-combination-of-luck-for-Democrats, Republicans will walk into 2015 controlling both the House and the Senate by reasonable margins. (As of writing, the Post's Election Lab figures that there will be 53 Republicans in the Senate and 243 in the House.) It will be the first time the GOP has controlled both chambers since the 109th Congress—and, if the Election Lab numbers hold up, the widest margin of control since 1929.
Below is a chart showing the percentage margin of majority in each chamber since 1869 (the point at which the Confederate states had rejoined the Congress). So if Republicans hold 53 seats in a 100-person Senate to the Democrats' 45, they have an eight percent margin of majority. (We excluded caucusing independents and, in the case of resignations and deaths, used the point at which the GOP had the highest membership.) And if they hold 243 seats in the House to the Democrats' 192, that's an 11.7 percentage point advantage—or a 19.7 percentage point combined majority in the Senate and House.