Here’s how Trump damage could hand Democrats control of the House

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.

Before the Trump Implosion, most pundits figured that 14 of those seats were leaning Democratic, 15 would likely go to the GOP and just nine were true "toss-ups." But that math has begun to change after Trump's downward spiral in national polls.

Handicapping House races, though, is a lot harder than other political contests because publicly available voter polls are few and far between. Most news organizations focus on national races or a handful of "battleground" Senate contests.

So most predictions about House races look backward at the last two cycles to see which way a district voted, and how wide the margin was between the two parties. Those predictions largely assume that voters will behave more or less the way they did in a "normal" election.

But this year's race has been anything but normal.

No matter how well a district has been gerrymandered, candidates still have to get the party faithful to come out and vote. And if the massive defection of Republicans sparked by rejection of Trump's candidacy spills over further down the ballot, some of the House seats considered "safely" Republican could be in play.

One way to find out is to look at the GOP margins of victory in the 2014 cycle. If Trump depresses the turnout by, say, 10 percent, those Republican House members that won by that margin — or less — could be in trouble.

If you add those seats to the contests that are already leaning Democrat, the GOP can still withstand a 10 percent defection rate, and hold the House with 223 of the 435 seats. With a 10 percent GOP defection rate, the math looks like this:

That math, of course, assumes that Democrats completely run the table, with none of the party's incumbents losing ground in upsets and all of the "toss-ups" falling in the Democratic side of the aisle. That's a major assumption.

Democrats also have to get their base of voters to the polls, in a year when Hillary Clinton faced a bruising challenge from Vermont Sen. Bernie Sanders. But while many of those voters had pledged to withhold their vote from Clinton, Sanders has endorsed the party's nominee and the rest of the party, including President Barack Obama and first lady Michelle Obama, have remained unified in their support of the ticket.

Trump, on the other hand, has lost the support of dozens of GOP senators, congressional members, governors and other party leaders, who have publicly stated they won't vote for him. Many Republicans, especially the conservative wing of the party, have taken strong issue with Trump's positions on traditional GOP issues.

Even with Trump's waning support among some Republicans, the power of the incumbency will protect much of the GOP hold on the House. But the Democrat's recent surge of hope in regaining control of the lower chamber isn't so far-fetched any more.

If that defection spreads much deeper than 10 percent, the change of control becomes much more likely.