Texas Republicans have their work cut out for them if they hope to maintain the party’s dominance there, according to the state’s senior U.S. senator.

Senate Majority Whip John Cornyn, a Republican who has been representing Texas in the U.S. Senate since 2002, said last year’s election battle between Republican Sen. Ted Cruz and Democratic challenger Beto O’Rourke should serve as a wake-up call for the state’s GOP because it revealed that Texas Democrats are determined to defeat President Trump – and take down other Republicans with him.

“I don’t think we can take for granted that Texas will be reliably Republican in the foreseeable future, unless we take care of our business,” Cornyn told the Dallas Morning News.

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“I don’t think we can take for granted that Texas will be reliably Republican in the foreseeable future, unless we take care of our business.”

— Senate Majority Whip John Cornyn

Democrats’ dislike for Trump was the key factor in why incumbent Cruz defeated O’Rourke by only a narrow margin, Cornyn said.

"President Trump was responsible for 100 percent of the turnout," Cornyn told the Morning News, referring to the November election in Texas. "Fifty percent turned out [for Cruz] because they wanted to support the president. Fifty percent turned out [for O’Rourke] because they wanted to defeat him.

“Effectively,” Cornyn said, “this was a referendum in some sense on [Trump]. That's still going to be a factor in 2020.”

"President Trump was responsible for 100 percent of the turnout. Fifty percent turned out [for Cruz] because they wanted to support the president. Fifty percent turned out [for O’Rourke] because they wanted to defeat [Trump]."

— Senate Majority Whip John Cornyn

By that year, more than a third of the U.S. electorate will be composed of nonwhite voters, according to a Pew Research Center analysis released Wednesday.

The trend tends to favor Democrats, who gained two U.S. House seats, plus 12 state House seats and two state Senate seats in November, the Morning News reported.

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Cruz captured less than 51 percent of the vote when he defeated O’Rourke in November. The Democrat, a congressman from El Paso, attracted more than 48 percent of the vote – a strong showing that has fueled speculation that he will seek the Democratic Party’s presidential nomination in 2020.

Despite his concerns about 2020 and beyond, Cornyn has remained solidly behind President Trump.

Last week, Cornyn joined a group of other Republican senators in urging Trump to send ISIS fighters captured in Syria to the Guantanamo Bay detention center in Cuba.

Also earlier this month, Cornyn applauded the president for taking his border wall push “directly to the American people” in a speech from the Oval Office.

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"There is a train of misery coming into the United States from these transnational criminal organizations that traffic in drugs and people and human misery," Cornyn noted of the border situation.

In October, Cornyn denounced those who were trying to blame President Trump for a series of pipe bomb packages shipped to prominent Democrats.

“These are some of the same people who encourage their own partisans to incivility and to confrontation and to encourage the mob that we saw during the Kavanaugh hearings,” Cornyn told Fox News’ Brian Kilmeade.