Looking to contain the damage, Democrats are pumping money into liberal congressional districts that were long thought to be safely in their column. Over the last several days, the Democratic Congressional Campaign Committee has directed resources to maintain seats in Hawaii and Nevada, both of which broke sharply for the president in 2012 — an indication of just how much the terrain has shifted against the party over the past two years.

Other unexpected races are suddenly in play. Some Democrats, for example, have begun to worry about the prospects of California Rep. Lois Capps, an eight-term congresswoman who is typically a lock for reelection but who now finds herself in a competitive race against Republican Chris Mitchum, a perennial candidate and the son of the late actor Robert Mitchum. In a sign of how seriously national Democrats are taking the threat, the DCCC is making a last-minute purchase of $99,000 worth of radio advertising in the Santa Barbara area to boost Capps, according to a committee aide.

Operatives from both parties expect Republicans to net five to 10 seats, which would give them some cushion heading into what’s expected to be a much more challenging 2016. Some Republicans, trying to tamp down rising expectations of even bigger gains, point out that a recently-redistricted congressional map has dramatically narrowed the playing field of competitive districts and limited potential pick-ups.