(CNN) It's become an old cliché that elections "come down to turnout." While that may be exaggerated in most places, it's definitely not in Texas. Democrats need voters who don't traditionally vote to cast a ballot this year in order to to win, and it's not clear that they will in 2018. This could cost Democrats on both the Senate and House sides.

A new Quinnipiac University poll of the Texas Senate race finds Republican Sen. Ted Cruz leading Democrat Beto O'Rourke by a 9-point margin. That's Cruz's best poll in over two months, and an increase from his 6-point advantage last month in the Quinnipiac University poll

Some of this change could be statistical noise, but Quinnipiac also changed the population it was polling between surveys. They went from registered to likely voters.

Not all voters actually cast a ballot in an election. That's why it's important to poll the subset of voters who actually will.

Most public pollsters shift to likely voters around this time as it becomes clearer who is going to vote in the midterm the closer we are to an election. That's especially the case for pollsters who don't rely on a voting list such as Quinnipiac to know how often people have voted in the past.

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