Texas Democrats longing for Beto O'Rourke or Julián Castro to abandon their presidential aspirations to run for Senate need to accept that it's not happening.

Both candidates have made it clear that they are running for president — for better or worse — and won't shift gears to take on Republican incumbent John Cornyn in 2020.

When he rebooted his presidential campaign last week, O'Rourke said he wanted to take the fight "directly to the source," President Donald Trump. That same day Castro told The Dallas Morning News that he would also remain in the presidential race.

Democratic presidential hopeful Secretary Julian Castro talked to Texas Working Families Party members during a candidate forum at the Village Bridge Center in Dallas on, Aug. 15. (Brian Elledge / Staff Photographer)

That's not good news for many Texas Democrats.

In a recent Emerson College poll conducted for The News, 46% of voters questioned said O'Rourke should drop out of the presidential race and run for Senate, while 25% said he should keep running for president and 29% weren't sure. About 51% of Democrats said O'Rourke should run for Senate instead of the White House.

About 41% of poll participants said they wanted Castro to drop out of the race and run for Senate, with 31% unsure and 29% saying he should stay put.

But there's always a chance for "someone else" to emerge.

The same poll showed that of the four major Senate candidates in the race at the time, 19% of respondents preferred "someone else," and 51% were undecided.

That means Democrats are not yet thrilled with the current crop of Senate contenders. The list includes former Air Force helicopter pilot and businesswoman MJ Hegar. She's been in the race for much of the year, raising about $1 million but only netting 10% support in the Emerson poll, far behind the leader, "someone else."

State Sen. Royce West of Dallas was third with 8%, followed by former U.S. Rep. Chris Bell of Houston at 7% and outgoing Houston council member Amanda Edwards at 5%. The campaigns of those three were launched in recent weeks.

Austin-based activist Cristina Tzintzún Ramirez, who entered the race last week, was not a candidate when the poll was conducted.

Ed Espinoza, executive director of the liberal advocacy group Progress Texas, said several of the Democratic contenders would be good candidates against Cornyn, once voters become engaged in the March primary contest. The Democrats in the Senate race are largely unknown to most Texas voters, though Bell was the 2006 Democratic Party nominee for governor.

"Most Texans won't tune into the Texas primary until the fall," Espinoza said. "Democrats are hungry to take a swing at John Cornyn. His numbers are soft."

Sen. Royce West (top left, smaller mugshot), D-Dallas, and (continuing clockwise) former U.S. Rep. Chris Bell of Houston, Houston City Council member Amanda Edwards and former Army helicopter pilot MJ Hegar of Round Rock in the field of those trying to unseat the longtime incumbent. (File/wires)

Cornyn's poll data does suggest some weaknesses.

Only 37% approved of his job performance, while 31% disapproved. The polls found that 33% of Texans were neutral or had no opinion.

A strong Democrat with crossover appeal could give him trouble. It remains to be seen if that candidate exists.

Cristina Tzintzun Ramirez is a Democratic Party candidate for Senate. (2007 File Photo / Austin American-Statesman)

That's why the primary season is important for Democrats. It serves as an early indication of the political viability of the potential nominee, as well as a preseason before the big matchup against Cornyn.

A contested primary will sap resources from the eventual winner, but the exercise is necessary to build name recognition for the candidate, particularly with a party's base looking to turn Texas blue.

Democrats haven't won a statewide race since 1994.

Cornyn has served in the Senate since 2003 and hasn't been pushed in a campaign since he beat former Dallas mayor and former U.S. trade representative Ron Kirk in 2002.

So much of the 2020 election will hinge on President Donald Trump's performance in Texas vs. a yet-to-be-determined Democrat.

If Trump underperforms, that helps down-ballot Democrats. If he matches or exceeds his 9 percentage point victory against Hillary Clinton in 2016, that bodes well for Cornyn and other Republicans on the ticket.

Last year O'Rourke, the former congressman from El Paso, ran a close race against Republican incumbent Ted Cruz and had the coattails to drag many down-ballot Democrats successfully across the finish line.

Right now the Democratic contenders for Senate need presidential coattails and won't be as lethal as O'Rourke was in luring voters to the polls and raising campaign cash.

That doesn't mean Cornyn can't be beaten, but it will be a different kind of Senate contest and obscured by the presidential race.

Meanwhile, the pining for O'Rourke and Castro to get into the Senate race continues, even with their recent statements.

Castro, the former secretary of Housing and Urban Development and former San Antonio mayor, has yet to qualify for the September debate at Texas Southern University in Houston.

O'Rourke is struggling in the polls as well.

"They say they're not running for Senate," Espinoza said. "But the filing deadline is not until December. There's a lot that can happen between now and then."