Texas Elections 2018 More in this series

Editor's note: If you'd like an email notice whenever we publish Ross Ramsey's column, click here.

Three federal judges in San Antonio are listening to arguments about what’s wrong and right about the state’s political districts. But you go to elections with the districts you have, and the current maps — the ones those judges might or might not decide to change — show some weaknesses and some strengths for incumbent politicians and parties in 2018.

Lots of political types have pointed to presumably Republican districts in Texas (and elsewhere) where voters chose Democrat Hillary Clinton over Donald Trump in last year’s presidential general election.

The takeaway, generally speaking, is that Republican members of Congress in those districts need to watch their backs since their futures depend on voters with wandering eyes.

That’s probably good advice, but it’s worth pointing out that those Republicans won their 2016 elections in districts where their party’s presidential candidate was losing. They are, by that definition, stronger than their party’s standard-bearer.

Democrats are buoyed by the idea that those incumbents won’t do as well against top-notch challengers; 2018 will be the proving ground for both of those theories.

Three Republican members of the Texas delegation — U.S. Reps. John Culberson of Houston, Will Hurd of Helotes and Pete Sessions of Dallas — won in districts where Clinton beat Trump.

Barely, it should be pointed out. While Trump finished 9.4 percentage points ahead of Clinton statewide, he lost in Hurd’s district by 3.5 percentage points and lost the other two by less than two percentage points.

Other Republicans who faced Democrats on the statewide ticket did considerably better, winning by more than 7 percentage points in both Sessions’ and Culberson’s districts. Voters in Hurd’s Congressional District 23 favored statewide Republican candidates, on average, by a scant 0.8 percentage points.

As it turns out, no one in the state’s congressional delegation serves in enemy territory; the Republicans are in districts where Republicans win, and the Democrats serve where Democrats win. Also, downballot Republicans tend to do better in elections where a governor’s race is in the center ring. Their margins slip in presidential election years.

In 2014, when Gov. Greg Abbott was elected, statewide Republicans in Texas beat statewide Democrats by an average of 22.4 percentage points. With Trump at the top of the ticket, that average slipped to 9.4 percentage points; with John McCain at the top in 2012, the Republican advantage in Texas was 14.8 percentage points.

That’s all good news for Republican incumbents seeking re-election in 2018, although as they say at the end of those investment commercials, past performance does not guarantee future results. Midterm elections often play out as referendums on presidents, and close races next year could reflect whatever Texans are feeling about Trump when they vote.

Hurd won the tightest race in 2016 (1.3 percentage points) and in 2014 (2.1 percentage points), both times against Democrat Pete Gallego of Alpine. CD-23 is the state’s only true swing district on the current political maps.

Unless, that is, the judges in San Antonio remake the lines; plaintiffs are suing the state for allegedly drawing intentionally discriminatory lines to suppress minority (and often Democratic) voting strength. The decision from those three judges could flip some of the state’s 36 congressional seats from one party to the other, make easy races more difficult, rejigger things without changing the partisan numbers — or change nothing at all.

If the maps don’t change, either party could win Hurd’s seat — that’s the history of that district. Anything else would have to be called an upset: Not impossible, but not the expected result.

Blame the maps. If the congressional race results had tracked the presidential results in Texas, there would be 22 Republicans and 14 Democrats in the state’s congressional delegation. In real life, there are 25 Republicans and 11 Democrats.

The candidates and the parties can nibble around the edges for slight changes in advantage each election, and each party’s primary voters can replace one partisan with another at will. But the political mapmakers have more real power over change and status quo, and right now, the mapmakers to watch are the three federal judges hearing the redistricting litigation in San Antonio.

When they’re done, you’ll have a pretty good idea which congressional races will be competitive.