WASHINGTON - As President Donald Trump embarked for El Paso on Monday to rally support for a border wall, Texas Republican John Cornyn sent out a personal message through his 2020 U.S. Senate re-election campaign:

“Texas stands with President Trump.”

For Cornyn, seeking a fourth term in the Senate, the message underscored some of the central challenges of his re-election bid: for better or worse, his fate is inextricably tied to that of a famously polarizing and unpredictable president, with whom he will share a ballot.

“As in the rest of my life, I don’t sweat too much the things I can’t control,” Cornyn said later in the week. “I look at the things I can control, and I can control my preparation for what I think will likely be a fairly serious opposition in 2020. The president is at the top of the ticket, and I believe he will be responsible for nearly 100 percent of the turnout, about half of the voters for him, and half against him.”

Cornyn, a former state Attorney General and Supreme Court Justice, knows that 2020 could be the most severe test of his time in the Senate, which began in 2002. He is the first to admit that Texas is not the GOP bastion it once was.

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Despite a generation of GOP dominance in the Lone Star State, demographic changes and urbanization have helped Democrats narrow that gap in statewide elections, culminating with Beto O’Rourke’s loss to U.S. Sen. Ted Cruz in November by less than three percentage points.

Only four years earlier, in the state’s last Senate election, Cornyn trampled relatively unknown dentist-turned investor David Alameel, while Republican Greg Abbott glided to victory by 20 points in the governor’s race against then-Democratic star Wendy Davis.

For Republicans, one of the first signs of trouble came in 2016, when Trump, riding a wave of anti-establishment fervor, carried Texas by only nine points against Hillary Clinton - an unaccustomed single-digit victory.

Suddenly, 2014 looked like eons ago.

“It was a pre-Trump era,” said Republican strategist Brendan Steinhauser, who ran Cornyn’s 2014 campaign. “The trends that we’ve seen in terms of the time that’s passed is that Texas has continued to urbanize, it’s continued to attract people from all over the country and all over the world, growing rapidly.”

“Many of those people are open to supporting Republican candidates,” Steinhauser continued, “but in this era of Trump’s Republican Party, it’s a different calculation.”

Steinhauser describes Trump as “the elephant in the room” who can nationalize state and local elections across America. But there’s also a home-grown factor that adds to Cornyn’s uncertainty: Beto O’Rourke.

Beto is Cornyn’s biggest threat. But will he run?

O’Rourke, the former El Paso congressman and current Democratic rock star, is known to be mulling a 2020 presidential bid. But many Texas Democrats see the Senate race as the perfect fallback position for the idealistic 46-year-old who raised a record $80 million for his challenge to Cruz.

Whatever O’Rourke decides, he is easily - and potentially the only - real threat to Cornyn in Texas. But for now, O’Rourke’s indecision simply adds a “Beto factor” to the list of unknowns for Team Cornyn.

“The degree of difficulty John Cornyn is going to have in 2020 right now I think very much rests in the hands of Beto O’Rourke,” said political scientist James Henson, director of the Texas Politics Project at the University of Texas.

While Cruz labeled O’Rourke “too liberal for Texas,” Democrats like the contrast of a youthful, relative outsider against a 67-year-old incumbent who earned his stripes the old-fashioned way: working his way patiently up the Senate GOP ladder.

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To many Texas Republicans, O’Rourke represents Cornyn’s worst-case-scenario. But some also see him as a one-off candidate that no other Texas Democrat can easily replicate. Next in the Democratic echelon are U.S. Rep. Joaquin Castro and his twin brother, former San Antonio Mayor Julián Castro. Many believe that Julián Castro’s own White House bid takes both brothers out of the Senate race.

Other than O’Rourke - who Texas Republican strategist Matt Mackowiak calls “a hundred-year flood” in Texas politics - that would seem to clear the decks for Cornyn.

“Whoever runs against Cornyn doesn’t start where Beto stops,” Mackowiak said. “They start wherever they are.”

For Texas Democrats, that means starting with an expected voter share in the high 30 to low 40 percentage points - the average electoral result pre-Beto.

Manny Garcia, executive director of the Texas Democratic Party, says that Cornyn has failed to strongly define himself after three terms in office, something he attributes to a lack of the sort of “big character” Texans expect.

Cornyn’s job approval ratings in Texas have hovered in the high 20s, though they spiked to 39 percent last October, a time of strong partisan feeling in advance of the midterm elections.

But whoever the Democrats put up against Cornyn, Garcia said, recent trends show that “it’s a single-digit race, no matter what.”

No room in GOP for complacency

Cornyn, a conservative in the traditional mold, does not elicit the same visceral reaction among Texas Democrats as Cruz, a hero of the tea party base. On the flip side, Cornyn doesn’t command the same fervor of the GOP activists.

“Cornyn is often damned with faint praise, compared to more recently elected state leaders like Ted Cruz and Greg Abbott,” Henson said. “Cornyn really is the product of a previous political generation of Republicans in the state, and he is sometimes greeted as such, particularly by more ideologically oriented Republicans - the conservative core of the party.”

But if the anti-establishment, anti-Washington, anti-compromise wing of the GOP finds Cornyn less attractive, Henson added, “less attractive does not mean rejected.”

Heading off any primary challenges from the right, Cornyn secured early endorsements this year from both Lt. Gov. Dan Patrick, a potential rival, and Cruz, who declined to endorse Cornyn during his senior colleague’s 2014 primary.

This year, expecting to have to break a sweat, Cornyn has built up an early $5.8 million campaign war chest and made clear that he’s taking nothing for granted in the general election.

“The message of the 2018 midterm elections was that you better be prepared, and that Republicans need to lose their complacency and realize that Texas is evolving and is no longer a reliably red state,” he said.

He’s also put together an all-star campaign team, including John Jackson, who ran Abbott’s 2018 re-election campaign, and former Texas GOP Chairman Steve Munisteri, a veteran of two years in the Trump White House.

Munisteri has long been a prophet of inclusiveness and diversity in the Republican Party, calling it not only a practical but a moral imperative, particularly as the Texas electorate grows younger and less white. Cornyn won an equal share or even a plurality of the Hispanic vote 2014, an achievement Munisteri notes “people forget” amid the buzz about the potential of Latino voters to deliver a “blue wave” in Texas.

But Munisteri recognizes that Trump has changed the political equation in Texas, much as in the rest of the nation. The key, he says, will be matching the Democrats’ anti-Trump verve, which cost Texas Republicans two congressional seats in 2018.

“Turnout, turnout, turnout,” Munisteri said. “I think the president is going to generate turnout in both parties… The challenge for Republicans is to be equally motivated.”

‘The voters of Texas know me’

For Cornyn, that will mean deftly navigating a relationship with Trump, who in terms of style, if not substance, comes from the opposite side of the Republican tracks.

Cornyn has lined up with Trump on tax cuts, regulatory rollbacks and conservative judges. But as a classic free trader, Cornyn has been wary of Trump’s protectionist trade policies. He’s also joined other top Senate Republicans in repudiating Trump’s decision to pull troops out of Syria and Afghanistan.

Like many Texans, Cornyn also has shown a much more nuanced grasp of border security, emphasizing the need for technology and personnel as well as physical barriers on the U.S.-Mexico boundary. He’s also raised questions about Trump’s threat to declare a national emergency to redirect federal spending on a border wall.

But in El Paso, where Trump was heavily criticized by local officials for overstating the city’s immigration and crime problems, Cornyn praised the president for “speaking the truth.”

Some analysts see little choice but for Cornyn to swing to the right as part of a strategy to consolidate the Republican base, which still loves Trump.

If that proves to be a tightrope walk, Cornyn believes he has the political capital in Texas to pull it off. “I think the voters of Texas know me,” he said. “Or at least enough of them know me well (enough) that they don’t confuse me with the president or anyone else.”

But if the Trump alliance has its downside in 2020, there’s still a sense that Texas is, after all, still Texas.

“I think Trump being on the ticket statewide helps Cornyn, relative to 2018,” Mackowiak said. “In certain areas of the state it won’t - in the suburbs potentially, in urban areas. But broadly speaking, I think it’s going to turn out voters that might not otherwise turn out.”

Then again, Trumpism isn’t always easy to predict. “We’ll see what Trump’s numbers are,” Henson said. “But I don’t expect that in the terrain of 2020 that Donald Trump will be a less polarizing or controversial leader of the Republican Party.”