Updated at 4:45 p.m.: Revised to include more information throughout.

Incumbent Republican Ted Cruz has opened a 9-point lead in his high-profile U.S. Senate race against Democrat Beto O'Rourke, according to a poll released Tuesday.

A survey of 807 likely voters by Quinnipiac University has Cruz with a lead of 54 percent to 45 percent over O'Rourke, the congressman from El Paso, with a margin of error of plus or minus 4.1 percentage points.

"The Texas U.S. Senate race between Sen. Ted Cruz and Congressman Beto O'Rourke, and Democratic hopes for an upset win there, have boosted talk of a Senate takeover," said Peter A. Brown, assistant director of the Quinnipiac University Poll. "These numbers may calm that talk. Congressman O'Rourke may be drawing big crowds and media attention, but Texas likely voters like Sen. Cruz better."

An August Quinnipiac survey showed Cruz leading 49 percent to 43 percent among registered voters.

"Any poll that shows you that far behind is significant," said Republican political consultant Bill Miller. "It's pretty damaging for Beto O'Rourke if it's among likely voters. Likely voters are consistent voters that will show up in November."

But Miller said it's too early to determine if O'Rourke has run out of steam.

"It's a long time before Election Day," he said.

The poll comes three days before the first of three debates between the nominees for Senate, paving the way for a potentially explosive clash as the candidates try to contrast themselves in front of Texas voters.

The first debate, sponsored by The Dallas Morning News, KXAS-TV (NBC5) and Southern Methodist University, will be held at 6 p.m. Friday at SMU. It will be streamed live on dallasnews.com and air live on NBC5.

Both candidates view the hourlong Dallas debate as an opportunity to reach a wider audience and improve their standing for the ultimate survey, the Nov. 6 election.

If the Quinnipiac poll is accurate, it foreshadows that O'Rourke will come up short in November, leaving Texas Democrats still in the political wilderness. The CBS 11/Dixie Strategies poll, another poll of likely voters released last week and conducted a week earlier than the Quinnipiac poll, had Cruz with a 4-percentage point lead, 46 percent to 42 percent. Other recent polls have focused only on registered voters.

A Democrat hasn't won a Texas statewide race since 1994, when George W. Bush won the race for governor and ushered in an era of statewide Republican dominance.

Governor's race

The Quinnipiac poll also showed the race for Texas governor is all but over.

Greg Abbott leads former Dallas County Sheriff Lupe Valdez by a 58 percent to 39 percent margin among likely voters.

Abbott has raised over $50 million for his campaign and has enjoyed big leads since the election season began. Valdez is mounting her Democratic Party challenge of Abbott on a shoestring budget, spending well under $1 million.

Race is a big factor in the contest. The governor leads 69 percent to 28 percent among white voters, while Valdez was cruising 83 percent to 16 percent with black voters.

Perhaps most disappointing for Valdez is her showing with Hispanic voters. Democratic operatives recruited her hoping she would help the party lure more Hispanic voters to the polls.

But despite being the first Hispanic sheriff elected in Dallas County, she trailed Abbott with Hispanic voters 49 percent to 45 percent. Abbott has said one of his campaign goals is to win the Hispanic vote, a group critical to growth of the Texas Republican Party.

In the Senate race, O'Rourke had better numbers than Valdez among Hispanics, who favored the El Paso congressman 54 percent to 45 percent over Cruz.

Overall, 62 percent of voters approved of Abbott's job performance.

"Gov. Greg Abbott is highly regarded by Texas voters and appears on the way to a landslide re-election," Brown said. "His lead over Sheriff Lupe Valdez is just under 20 points among likely voters, and the governor gets good grades on his job performance."

The great hope for Texas Democrats involves O'Rourke. And his path to victory entails expanding the Democratic Party base vote while appealing to Republicans dissatisfied with Cruz or President Donald Trump.

But O'Rourke is struggling to get crossover support. The poll says Cruz is backed by a commanding 94 percent of Republicans, with the same percentage of Democrats supporting O'Rourke. Among independents, 51 percent favored O'Rourke, and 47 percent liked Cruz. O'Rourke's favorability was divided, with 43 percent viewing him favorably and 42 percent unfavorably.

Because Cruz enjoys a structural advantage in a Republican-leaning state, his path to success in November is to simply make sure conservative voters show up at the polls. His approval rating was 53 percent to 44 percent, with a similar favorability among likely voters.

The Cruz-O'Rourke showdown is the nation's costliest Senate contest, and O'Rourke has defied expectations by blistering past the incumbent in fundraising. In the second quarter of 2018, he raised $10.4 million, compared with Cruz's $4 million. He has raised $10 million more than Cruz overall.

Ads playing a role

The poll was conducted while both candidates' television ad campaigns were taking off. O'Rourke has been running a spot about his outreach to all of the state's 254 counties and the need for Texans to come together.

Cruz has been running ads that criticize O'Rourke's support of NFL players who kneel during the national anthem to protest the shooting of unarmed black Americans by white police officers.

The issue has stormed to the center of discourse in the race, but the poll found that likely voters view immigration as the most important issue, at 27 percent. Health care followed at 21 percent, with the U.S. Supreme Court at 16 percent and the economy at 15 percent. Cruz and O'Rourke have vastly different stances on these and most other issues.

Cruz won his seat in 2012 with a 16-point margin. In 2014, Sen. John Cornyn won a third term by 27 points, though both faced much weaker opponents than O'Rourke. Trump edged Hillary Clinton in Texas by 9 points.

The poll also found Texas' likely voters split evenly on Trump's job rating, with 49 percent approving and disapproving. Of Republicans, 86 percent approved and 12 percent did not, compared with 93 percent disapproval and 6 percent approval among Democrats.