Washington (CNN) 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.

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