The first official primary for the 2018 congressional elections takes place Tuesday: Texas voters go to the polls for the nation's first contest to decide each party's nominees for November.
Both parties will undoubtedly read the tea leaves when the polls close, putting the results under a microscope as the midterms get underway. For Democrats, for instance, there are some battles between the establishment and Bernie Sanders wings of the party.
But the larger importance of Texas is likely whether (or how far) it moves from the GOP on Election Day in November.
There is a reason Texas Democrats are so jazzed at the moment, and Republicans are sweating, even if most handicappers predict the state as a whole will remain in the GOP column for the time being.
The changing makeup of the Texas electorate is making for some unpredictable results in congressional races compared with years past -- and when you add a president as polarizing as Donald Trump, the uncertainty factor is further heightened.
That's because Trump's comparative weakness in traditionally strong Republican Texas districts was striking in 2016, and it's helping fuel Democratic enthusiasm that certain Texas House seats are primed to be flipped if they can get their voters to the polls.
In fact, Democrat Hillary Clinton successfully flipped voters in three Texas districts carried by Republican Mitt Romney in 2012 (two of them also went for Republican John McCain in 2008). Majorities supported her in two suburban districts around Houston and Dallas held by Republican Reps. John Culberson and Pete Sessions.
The third she flipped was the geographically enormous 23rd District, which covers a swath of West Texas along the Mexican border. That seat, held by Republican Rep. Will Hurd, is the scene of a split Democratic battle in Tuesday night's primary, as military veteran Gina Ortiz Jones, former federal prosecutor Jay Hulings and former Agriculture Department official Judy Canales face off for the chance to take on Hurd in November.
CNN has already rated the three districts "Key Races" heading into the fall, among the nation's most competitive to watch.
But an analysis of election results also shows that even in certain Texas districts Trump won, he did so by much lower margins than Romney or even McCain.
These House districts extend through the heart of Texas' population centers, almost making an L-shape down from Dallas in the north to Austin and over to Houston near the coast.
In four of the districts, Trump won by only single digits, compared with Romney's haul of 20 to 30 percentage points four years earlier.
Take Republican Rep. Kenny Marchant's district near Dallas: Romney won it by 22 points and McCain by 17 points; Trump barely cleared a 6-point victory over Clinton.
In the Texas 22nd near Houston, held by Rep. Pete Olson, Romney carried the day by 25 points and McCain by 21; Trump won by just under 8 points.
In another five Texas districts, Trump won by less than 15 points, which sounds impressive enough until you realize Romney won by significantly larger margins (Romney carried Texas 3rd Congressional District, held by Sam Johnson, which is north of Dallas, by 30 points. Trump received less than half that, only 14 points above Clinton.)
Trump's hardline immigration policies and his derisive comments about Latinos, Mexicans in particular, also make the state's demographics worth examining. If Latinos were to turn on the President en masse and voice their frustration against congressional and state-level candidates in November, Texas would be primed to be ground zero for such an upswell.
That said, it's worth noting that CNN exit polls in 2016 found Trump carried 34 percent of the Latino vote in Texas, slightly above his showing among Latinos nationally, at 28 percent.
While nationally, the percentage of the US population identifying as Hispanic is about 17 percent, in Texas that figure is nearly 39 percent, with the largest share of Mexican origin, according to the US Census American Community Survey's five-year estimates released in 2016.
There are now 47 counties in Texas with majority Latino populations. That's up from 40 compared with the previous American Community Survey estimates from 2011.
Eighty-three of Texas' 204 counties are at least a third Hispanic, and some of them have seen considerable growth in their Hispanic concentrations: in the neighborhood of 4, 5, 6 percentage points of growth compared with the 2011 numbers.
What's more, that number has only been growing, with all signs pointing to a continued increase in the coming years and decades.
The Texas percentage of Hispanics rose from 37.2 percent to 38.6 percent during the same period, out of a state population of nearly 27 million people.
All that has Texas politicos wondering whether the long-standing Republican hold on Texas may slip in the coming years. And whether the 2018 midterms could deliver some surprises.
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