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Unpopular presidents regularly get their parties clobbered in midterm elections, but Texas Republicans have a couple of layers of political insulation. Donald Trump is still popular with the party’s voters, and Texas Democrats would have to have an unusually strong year to win big even if there’s a Trump slump in 2018.

When Texas Republicans won the last round of state elections in 2014, the margins of victory were almost as important as the victories themselves.

In contested statewide races, the average Republican candidate finished 13 percentage points ahead of the average Democrat.

To win in an environment like that, a Democrat would have to outperform the rest of his or her ticket by a huge margin.

Of course, some Democrats won, but not statewide and not in districts that performed like the rest of the state. Those who won did so in districts drawn to favor Democrats or, more accurately, in districts where Republicans couldn’t legally configure the maps to favor their own candidates.

There are a number of congressional and legislative districts in Texas where Hillary Clinton beat Donald Trump in 2016. National apparatchiks from both parties have their eyes on U.S. Reps. John Culberson of Houston, Will Hurd of Helotes and Pete Sessions of Dallas — Republicans seeking re-election where Trump was weak.

That’s interesting, but so is this: In Culberson’s district in 2014, Republican Greg Abbott beat Democrat Wendy Davis by 21.8 percentage points. The spread in Hurd’s district was 13.8 percent; below the state average of 20.4 percentage points, but still formidable. In Sessions’ district, Abbott beat the Democrat by 15.8 percent.

Texas Democrats and their candidates weren’t completely responsible for that performance; they were running against the political winds in a midterm election during the Obama administration. If you flip the logic, lots of Democrats are hoping Trump will do for Republican contestants what Obama did for his.

In that sense, 2018 potentially provides a clean test of where the parties stand. Texas voted against Obama twice, and thumped his side in both of his midterm elections. And Texas was relatively kind to George W. Bush, the Texas president who preceded him.

Trump’s a break from all of that. Still, Texas Democrats have a lot to overcome, and doing that will require locating a standard-bearer to run well enough against the Republicans to attract voters to the polls.

What they’re hoping for is something like the 1990 election, which was a big break for Republicans, who pinned their hopes that year on Midland oilman Clayton Williams Jr. He lost, famously, to Democrat Ann Richards. But U.S. Sen. Phil Gramm won re-election and the Williams-Richards race was close enough to get a couple of down-ballot Republicans — Kay Bailey Hutchison and Rick Perry — past their Democratic rivals.

Those two wins — Hutchison as state treasurer and Perry as agriculture commissioner — were key to the eventual Republican takeover of Texas state government.

The loudest part of the election was the governor’s race, but the wins came elsewhere on the ballot.

Texas Democrats haven’t been able to put that formula together. They’ve certainly tried, running South Texas oilman Tony Sanchez Jr. against Perry in 2002, former Houston Mayor Bill White against him in 2010 (the year Perry beat Hutchison in a GOP primary) and Davis in 2014. The odd year out was noisy enough, with Perry facing Democrat Chris Bell and independents Carole Keeton Strayhorn and Kinky Friedman in 2006.

None of those races got any other statewide Democrats close enough to snag a victory. But this kind of thinking is what has so many eyes on the race for U.S. Senate between incumbent Republican Ted Cruz and Democratic challenger Beto O’Rourke, a congressman from El Paso. It’s the statewide race getting the most attention to date, both inside and outside of Texas. Cruz, after an unsuccessful run for president and an attention-seeking first term in the Senate, is a national figure. He remains popular with Texas Republicans and unpopular with the state’s Democrats — a perfect figurehead for a big political race.

O’Rourke has never run statewide, but has put together a voter-charming road-trip candidacy that has generated a lot of attention, news coverage and small donations to his campaign. It’s got a lot in common with the campaign Cruz ran as an upstart candidate in 2012.

It’ll be interesting and, perhaps, competitive. Maybe the president’s ratings will have an effect. And the rest of the people on this year’s ballot — no matter their party — will have something more than a sporting interest in the outcome.

The other Republicans on the ticket don’t want to end up like Jim Hightower or Nikki Van Hightower, the losers in those two 1990 upsets.