To which some Democrats quickly respond: What about Republicans?

Republicans, they point out, stood firmly against most anything Barack Obama did for much of his presidency, and while they didn't unseat him in 2012, they won back the House in 2010, the Senate in 2014, and after 2016 they're in as powerful a position as they have ever been. Call it what you want — “obstruction” or “principled opposition” — it seems to have worked out quite well for the GOP.

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But that's not a sure sign that it will also work for Democrats.

The reason I say this is polarization in this country favors Republicans more than Democrats, at least when it comes to Congress. Republicans have something of an inherent advantage in both the House and Senate, and polarization helps reinforce those advantages these days.

Why? There are simply more red states and more red congressional districts. Republicans took over the House and Senate in recent years largely because they knocked off some of the final hangers-on among Democrats in conservative-leaning places. It first happened in the South; then it spread to Appalachia and the Midwest.

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Thanks to that trend and that Republican-controlled state legislatures drew party-friendly U.S. House maps in many key states before the 2012 election, a straight-partisan vote for Congress pretty much ensures a Republican majority.

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The 2016 election is a good example of this. Trump, as everyone knows, lost the popular vote by two full points, 48 percent to 46 percent. But despite that loss, he actually won 230 out of 435 congressional districts, compared with 205 for Hillary Clinton, according to numbers compiled by Daily Kos Elections. And in the Senate, he won 30 out of 50 states.

So basically, 53 percent of House districts are Republican and 60 out of 100 senators hail from red states, according to the 2016 election results (in which the GOP, again, lost the popular vote).

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The question from there becomes how much — and how — Democrats will need to overcome this inherent disadvantage.

The median House district in this country in the 2016 election was Virginia's 2nd District, which went for Trump by 3.4 points. Democrats hold just five districts that went stronger for Trump than that median district — a reflection of our polarization and how predictably these districts tend to mirror the national vote. So Democrats would need to win just about every district that went for Clinton or narrowly for Trump.

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Republicans also have more districts “in the bag,” so to speak. Trump won 186 districts by double digits, compared with 171 for Clinton. And he won 211 districts by five or more points, compared with just 185 for Clinton.

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So Trump won more districts by at least five points than Clinton won overall, and he won more districts by 10 points than Clinton won by at least five. If we consider every district decided by less than 10 points in 2016 to be a battleground, Democrats need to win more than 60 percent of them to win the House majority back. And if you define the battleground more narrowly as every district decided by five points or fewer, Democrats need to win 85 percent of them.

What a lot of people don't realize about the Republicans' big wins in 2010 and 2014 is that they didn't really penetrate a whole lot of Democratic territory. Here's what I wrote when there was some chatter about Democrats retaking the House last year:

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... in the big GOP wave of 2014, Republicans only took over four districts that leaned toward Democrats , according to the Cook Political Voting Index (PVI). Were Democrats to win back the House this year, they would likely have to win a dozen or more seats that clearly lean toward Republicans, just by virtue of how friendly the map is to Republicans (both because of natural partisan sorting and gerrymandering). Republicans have an inherent advantage in holding the House that serves as essentially a sand dune beating back whatever wave Democrats can produce.

And that's even more the case in the Senate, where Democrats' path back to the majority in 2018 is difficult, to say the least. Given the states that are holding elections, Democrats will need to reelect every Democratic senator in big Trump states like Indiana, Missouri, Montana and North Dakota, while also stealing GOP seats in Arizona, Nevada and a heavily Trump state like Nebraska, Tennessee and Texas. Are they really going to do that if they go against everything Trump does?

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What got the GOP over the top in 2010 and 2014 was largely nailing down districts and states that, in a strictly partisan world, would have been theirs in the first place. Being partisan in Congress seems to have helped them accomplish that task.