The Republican primary to replace retiring U.S. Rep. Pete Olson, R-Sugar Land, is not the only jam-packed GOP nominating contest this cycle in Texas — but it may be the most consequential.

The 15-way primary is easily the most competitive one for an open Texas seat that national Democrats are targeting, and with just a few days left, attention is centering on who can emerge from a top tier of candidates to qualify for a virtually guaranteed runoff. The most serious contenders include Pierce Bush, the grandson of the late former President George H.W. Bush; Fort Bend County Sheriff Troy Nehls; and Kathaleen Wall, a self-funding GOP activist-donor who ran unsuccessfully for a neighboring congressional district in 2018.

“Oh, hallelujah, let me tell you — this is a big deal for us,” said Gary Shrum, a Brazoria County GOP activist who sees his county's favorite-son candidate, former Border Patrol agent Greg Hill, as Republicans’ best bet in November.

The groundwork was laid for the primary two years ago when Olson won reelection by a smaller-than-expected 5-percentage-point margin against Democrat Sri Preston Kulkarni, making the seat a national Democratic target this cycle. Kulkarni, a former diplomat, decided to run again, and Olson passed on a reelection campaign, creating a high-stakes vacancy in the heart of the politically changing Houston suburbs. Olson, who Nehls threatened to challenge in 2018, endorsed Bush for the seat last month, calling him the best Republican to keep the diverse district red in November.

"This is the most diverse county in all 254 and if someone thinks they're going to go after the Ronald Reagan Anglo community and win this deal, they'll never make it," said Craig LeTulle, a longtime GOP activist in Fort Bend County. Any serious contender, he added, has to be fishing for votes beyond what historically has been Republicans" "white little pond."

The general election has loomed large over the primary as Republican after Republican pitches himself or herself as the the best contender to take on Kulkarni, who faces four primary opponents Tuesday but remains the favorite for his party's nomination.

"[Democrats] do not want to see me get through this primary because they understand ... I am the biggest threat to Sri Kulkarni," Nehls said at a January forum while pitching his deep relationships in the district, professing confidence he was the only candidate at the event who could locate places like the Maryam Islamic Center in Sugar Land or the local BAPS Hindu temple.

To win in November, "you're gonna have to be someone who reaches out to all corners of the district," Bush said in an interview.

Bush, the CEO of Big Brothers Big Sisters Lone Star, has made regular trips to mosques in the districts. He was the only one of the three leading candidates to participate in a forum held earlier this month by the newly formed Indo-American Conservatives of Texas. And on Friday night, the campaign had a meet and greet with Venezuelan and Turkish communities that it said drew 200 people.

But before the Republicans can fully harness that outreach for November, they have to get through a blockbuster of a primary that has gotten so large — and filled with heavyweight names — that a couple Republicans who had been running for months changed their minds and decided not to file shortly before the December deadline, wary of the gauntlet they would have to endure.

Wall has put nearly $4 million of her money into her campaign, which comes after she spent $6 million of her wealth running in 2018 for the 2nd Congressional District, where she narrowly missed a runoff. Bush has raised over $1 million in contributions since entering the primary in early December. And Nehls has raised considerably less but enjoys high name ID as he finishes a second term as sheriff of a county that makes up nearly two-thirds of the district population.

Nehls is the "prohibitive leader" heading into a likely runoff, said Chris Elam, a former Texas GOP spokesman who lives in Fort Bend County but is not actively supporting any candidate. "His personal brand, his political brand — it's on fire in Fort Bend County."

The question to many Republicans watching the primary is who finishes behind Nehls. As LeTulle put it, Bush "has some momentum and Wall has some 'mo' — money."

Nehls, who waited to launch his campaign so that he could keep his job as sheriff, is leaning hard on his law-and-order experience, branding himself "one tough sheriff" and using a campaign logo featuring his head with a sheriff's hat on it. His ads boast that he has "locked up over 2,500 criminal illegal immigrants" as sheriff.

That issue has fueled arguably the most contentious conflict in the primary. Wall launched a TV ad about a month ago slamming Nehls as soft on illegal immigration, in part based on his opposition to a couple of the more severe elements of the "sanctuary cities" ban" that Gov. Greg Abbott signed into law in 2017. Nehls was otherwise supportive of rooting out sanctuary cities — jurisdictions that decline to cooperate with federal immigration authorities — and angrily pushed back against the Wall attack, claiming she misrepresented his broader record of fighting illegal immigration.

"Others may talk about being strong on illegal immigration, I've done it as Sheriff for seven years," Nehls said in a statement for this story. "I've been on the record supporting Trump's border wall for years—not just adopting the position as a candidate for Congress."

Bush too has put illegal immigration front and center in his campaign, using his debut TV commercial to declare Democrats "want open borders and it's dangerously naive."

Perhaps more than any other candidate, Bush's campaign has been an exercise in trying to square his belief in a more welcoming, empowering conservatism with the modern-day necessities of running in a Republican primary— full-throated support for Trump, dire warnings against socialism and, yes, a hard line against illegal immigration.

Fulshear Mayor Aaron Groff, who recently endorsed Bush, said in talking with the candidate, he gained an "understanding [of] his tough stance on border security but at the same time his compassion for those who have come here legally, refugees." That nuance, Groff said, is "absolutely" necessary in a district like the 22nd.

"We are growing so fast, we’re so diverse ... and we continue to diversify," Groff said. "I think it’s critical to have an understanding of that."

Then there is the Trump factor: a major theme in all of the crowded GOP primaries on the ballot Tuesday, but uniquely so in the 22nd District as the Bush name continues to invoke the family's two former presidents and their disdain for the current occupant of the Oval Office. Shrum said the Bush name is "not as influential in a more conservative county like Brazoria County as it used to be."

At the start of his campaign, Pierce Bush said he was running to be a "team player" for Trump in Congress, and he has continued to argue the president's policies have strongly benefited the 22nd District.

"It’s easy to align yourself with the president's policies and be an advocate for them, and I’m strongly in that camp," Bush said in the interview. "I think the president’s gonna be reelected and I think people will vote for him because his policies are good for the American people.”

But Bush also has at least one story in his background that undermines his embrace of the president: He marched in a January 2017 protest against the president's travel ban targeting several Muslim-majority countries. Asked about the ban in a TV interview earlier this month, Bush seemed to defend it, saying one of the president's top responsibilities is to "protect our country's national security." Bush disputed that Trump is "anti-Muslim," instead calling him "pro-immigration."

Wall is running the most overtly pro-Trump campaign, airing TV ads staged at the border in which she says, "It is going to take this Wall to build a wall." It is one of several spots — others emphasize her faith and rural upbringing — that Wall is using her millions to flood the airwaves with, much like she did during her 2018 campaign.

Wall's consultants include two of Trump's political advisers, Justin Clark and Bill Stepien.

Nehls is also tightly aligning himself with Trump — and has a high-profile incident to back up his support. He garnered national attention in 2017 for raising the possibility of disorderly conduct charges against a local driver with an explicit anti-Trump bumper sticker.

Trump has stayed out of the primary for the 22nd District, even as he has endorsed in a few other competitive congressional primaries across the state. At a campaign event earlier this month, Hill said he had talked to a White House political affairs staffer who "intimated that President Trump knows about the candidate in this race that marched against him" — an unmistakable to reference to Bush.

"President Trump could possibly get involved depending who is in the primary," Hill said.

The White House declined to comment on Hill's remarks.

As Wall's spending has soared, both Bush and Nehls have received modest outside support. The pro-Bush super PAC Texans Coming Together has dropped $138,000 in the primary, all in support of Bush. A pro-Nehls super PAC, One Tough Sheriff PAC, has spent $93,000, including $67,000 backing Nehls and $25,000 opposing Wall.

Both Bush and Wall are also up against questions about their local roots after moving into the district to run. Bush, who had initially looked at running in the nearby 7th District, says his nonprofit work has made him no stranger to the greater Houston area.

All three of the leading candidates can expect amplified attacks in a runoff, especially Nehls. He was fired from the Richmond Police Department in 1998 for a long list of violations, including destroying evidence, a chapter he has shrugged off as small-town politics that did not prevent him from rising to the sheriff's office.

Some of the lesser-known candidates have been sounding the alarm about Nehls for months.

"[Kulkarni]'s a very, very sharp man, and we have got to have our best foot forward," said Jon Camarillo, a retired Marine. "With the sheriff, he just brings too much baggage with him."