Democrats have a lot working in their favor this election cycle in their effort to take back the House.

Their voters turned out in proportionally greater numbers than Republicans in recent primaries and special elections. Polls show voters favor a Democrat over a Republican representing them in Congress. Republicans are retiring at historic numbers. Donald Trump is president, and a historically unpopular one.

But there's one outsize hurdle standing in the way of Democrats' sudden popularity: gerrymandering.

After the 2010 Census, Republicans controlled enough state legislatures to draw new electoral lines in four times as many districts as Democrats did. And Democrats have been locked out of power in some swing states ever since. A new report finds they might not make it back to power without lines that favor them, and Democrats don't have a reasonable chance to control the line-drawing process until after the 2020 Census.

new report from the Brennan Center for Justice calculates that Democrats are going to have to win the popular vote by a historically large margin — an estimated 11 percent —  to overcome Republican-drawn districts that were designed to keep them out. Winning by such a large margin is something no party has done in decades.

From the report, which published last week:

To attain a bare majority, Democrats would probably have to win the national popular vote by nearly 11 points. Neither Democrats nor Republicans have won by such an overwhelming margin in decades. Even a strong blue wave would crash against a wall of gerrymandered maps.

We should note that the Brennan Center opposes gerrymandering. It is a nonpartisan public policy institute, and it takes sides on public policy it thinks is best for democracy. Lately, courts have been agreeing with them that drawing electoral lines favoring one party over another is or might be unconstitutional.

But not everyone agrees with the Brennan Center's conclusion that gerrymandering will be a nearly insurmountable hurdle to Democrats' chances this November to win back the House. Yes, gerrymandering is something Democrats need to overcome, but it may be one they can — and have in the not-too-distant past — overcome.

Democrats probably only need to win the popular vote by 7 percent to obtain a bare majority in the House, calculates David Wasserman, the House editor for the nonpartisan Cook Political Report, which analyzes congressional races. And while it may not seem like much of a difference, winning 7 percent of the popular vote is a more doable number than 11 percent in politics. Both parties have performed that well in wave elections.

Self-segregation is another reason Democrats have been locked out of power in the House. Democrats tend to live in more urban centers, Republicans in more rural areas. That means there is a high concentration of Democrats in cities, which already tend to be represented by Democrats.

Democrats are, Wasserman put it, “wasting millions of votes in districts they already hold.”

Also, even if Democrats do need to win the popular vote by a historically large margin to take back the House, there is some very early evidence they might be able to do that.

In Pennsylvania this month, a Democrat won a congressional special election in a district President Trump won by 20 points. There are nearly 120 more competitive congressional districts than Pennsylvania's 18th, according to a ranking by Cook, and Democrats only need to net 23 to take back the House.

But winning the House in 2018 doesn't guarantee that Democrats will keep it in the following election.

To regain map-drawing power after 2020, Democrats need to focus on winning at the state level rather than the congressional level. In most states, it's state lawmakers who draw electoral districts every 10 years after a new census comes out.

And regaining power in the states is much more of a hurdle for Democrats than winning back the House. Republicans control 67 of 99 legislative chambers. When you layer on governors, Republicans control state government in more than half of the country, and at least some of it in about 80 percent of states.

Democrats are in such a deep hole when it comes to power in the states that some political scientists think even a wave election this November won't be good enough to keep them there. Nor will a wave election in 2018 and a so-so one in 2020. In the end, Republicans are more likely than not to control a majority of states and their mapmaking processes, and that means they'll have the ability to lock Democrats out of the House for another decade.