For Democrats, winning back the House won't be easy. Here's how it might happen.

The ongoing implosion of the Donald Trump campaign, after release of a tape capturing the Republican presidential nominee's demeaning remarks about women along with claims by women that they had been groped by the candidate, has touched off widespread concern about the fallout on the rest of the GOP ticket.

This week, dozens of down-ballot candidates running for House and Senate seats have repudiated the standard-bearer in a wave of defections not seen in decades. House Speaker Paul Ryan has effectively told down-ballot Republican candidates to put their own campaigns first.

Ordinarily, changing control of the House is an uphill battle for the party out of power. That's because of the longstanding power of the incumbency brought about by decades of redistricting. Over the years, both parties have rewritten the boundaries of their districts to solidify their support by drawing a line around highly concentrated pockets of voters, insuring that congressional districts stay in each parties hands.

In many districts, the daunting reality of the long odds in unseating an incumbent also makes it harder for the party out of power to field strong candidates. Even when an incumbent retires or moves on to run for Senate, a party's hold on a given district can be tough to break. This year, of the 40 House seats that are open to newcomers on both sides, more than half are considered "safe" for the party in power.

As a result, party control typically shifts each two-year cycle in only about 15 percent of congressional districts. That means the battle for control is fought in just a few dozen contested seats. This time around, the breakdown favors Republicans, with just 38 of the 435 seats considered real contests.