SOME VISUAL CONTEXT....It's not unfair for Republicans to characterize the midterms as "historic," at least as far as the House is concerned. The GOP not only made massive gains, they'll enjoy their largest House majority next year in more than six decades.
But as is my habit, I thought I'd put the elections in charts. Here, for example, are the midterms of the modern political era, with losses per cycle for the president's party (blue columns represent Democratic presidents, red columns represent Republican presidents).
You'll notice, of course, that the 2010 midterms were the worst for any incumbent president's party since FDR's drubbing in 1938. Also note, this chart only reflects the results of yesterday's elections through the available data. (Several House races have not yet been called, so the results may yet get slightly worse for Dems, though it does not appear they'll match the 1938 totals.)
On the other hand, Senate losses for the White House's party -- at this point, it looks like Dems have lost six seats from their majority -- were fairly mild by modern standards. Indeed, while Obama's House losses were greater than Clinton's, Clinton's Senate losses were greater than Obama's.
In some cases, the results are a little misleading, in part because the president's party went into a midterm cycle with smaller congressional minorities, and didn't have much further to drop.
That said, I think the visual context helps.
Also, the senate losses in 1994, when from eight on election day to 10 wehn Richard Shelby and Ben Nighthorse Campbell switched to the GOP.