HOUSTON — Beto O’Rourke entered the race for Texas governor on Monday, challenging an ultraconservative and well-financed two-term Republican incumbent in a long-shot bid to win an office Democrats last occupied in 1995.

The arrival of Mr. O’Rourke immediately set the stage for a pitched political showdown next November over the future of Texas at a time when the state — with its expanding cities and diversifying population — has appeared increasingly up for grabs.

Mr. O’Rourke, the former El Paso congressman and 2020 Democratic presidential candidate, has been a darling of Texas Democrats and party activists since his run against Senator Ted Cruz in 2018. Though he lost the Senate race by nearly 3 percentage points, the fact that Mr. O’Rourke came close to unseating the incumbent Republican senator transformed Mr. O’Rourke into a national figure and convinced many Democrats that the state was on the cusp of turning blue.

His campaign hopes to rekindle that enthusiasm as it attempts to unseat the Republican governor seeking re-election to a third term, Greg Abbott. One recent public poll found Mr. O’Rourke nearly tied with Mr. Abbott in a hypothetical match up, and another showed him losing by nine.

“Those in positions of public trust have stopped listening to, serving and paying attention to — and trusting — the people of Texas,” Mr. O’Rourke said in a video announcing his campaign that was released on Monday. He contrasted the “extremist policies” of Texas Republicans that have limited abortion and expanded gun rights with positions that he said he would support, including expanding Medicaid and legalizing marijuana.

And the video sought to recapture the anger felt by Texans when the state’s power grid failed in February. “It’s a symptom of a much larger problem that we have in Texas right now,” he said.

But Democrats have also seen their story of political change in Texas complicated by the results of the 2020 election.

Former President Donald J. Trump carried the state by nearly six points and gained ground for Republicans among Hispanic voters in the Democratic stronghold of the Rio Grande Valley. Republicans also held the State House of Representatives despite a concerted effort by Democrats to flip control. And Republicans have had an electoral lock on the governor’s mansion that has stretched for nearly three decades. The last Democrat to serve as governor was Ann Richards, who won election in November 1990 and was in office from January 1991 to January 1995.

The 2022 race will take place against a national backdrop that favors Republicans, including an economy still struggling to rebound from the pandemic and a Democratic president whose popularity has been sinking. And after his own failed presidential run, Mr. O’Rourke faces the challenge of demonstrating to Texas voters that he is focused on the state’s issues and not on the national spotlight.

His advisers appeared to be aware of the need to remind voters of the actions Mr. O’Rourke has taken in Texas, particularly after the winter storm sparked the devastating blackout in February. Mr. O’Rourke solicited donations for storm victims, organized wellness checks for seniors and delivered water from his pickup truck.

His organization, Powered by People, has also helped to register voters — nearly 200,000 since late 2019, according to the campaign — and Mr. O’Rourke raised around $700,000 to support Democrats in the Texas House after many fled to Washington to block a restrictive new voting measure that ultimately passed.

He has also used his platform to push for pandemic-related public health measures like those backed by local Democratic leaders in Texas, a contrast to Mr. Abbott, who has banned mandates for masks or vaccines.

The message of the campaign, his advisers said, is that Mr. O’Rourke has been there for Texans while Mr. Abbott has put his own political ambition and the demands of Republican primary voters over the needs of ordinary people.

In the video, Mr. O’Rourke, who speaks fluent Spanish, made his announcement from the majority-Hispanic border city of El Paso, where he grew up and now lives.

Democrats had been urging Mr. O’Rourke to jump into the race for months, and he had begun to strongly consider doing so by late summer as he called around to Democratic leaders in the state.

With the election a little less than a year away, no other major Democrat has jumped into the race, leaving Mr. Abbott’s advisers to consider a range of messages to attack Mr. O’Rourke as too extreme for Texas. They are likely to focus on comments he made about guns and the border wall during the 2020 Democratic presidential primary.

“Republicans didn’t need a lot of reason to turn out and have intensity, but this is going to juice it,” said Matt Mackowiak, an Austin political consultant who is the chairman of the Republican Party in Travis County, referring to Mr. O’Rourke entering the race. “It’s going to be kryptonite for Democrats in suburban areas, and it’s going to be rocket fuel for Republicans in rural areas.”

Well before Mr. O’Rourke’s announcement, the governor’s campaign began releasing digital ads featuring montages of those statements, including one from a 2019 debate that has come to define what some Texas political observers see as Mr. O’Rourke’s uphill battle in the state.

“Hell yes, we’re going to take your AR-15, your AK-47,” Mr. O’Rourke said, to applause from the crowd.

At the time, his presidential campaign promoted a shirt with those words. His Texas campaign is likely to take a different tack. Aides said Mr. O’Rourke is calling for an expansion of background checks.

Despite having never won statewide in Texas — no Democrat has since 1994 — he has remained one of the few Democrats with enough fund-raising prowess and statewide campaigning ability to take on Mr. Abbott, a former Texas attorney general who has overseen a dramatic turn to the right in the state and commands a campaign war chest of nearly $60 million.

The eagerness among Democrats to find someone to challenge Mr. Abbott led to interest in seeing the actor Matthew McConaughey jump into the race. Mr. McConaughey has said coyly that he is “measuring” doing so but has yet to make any announcement.

For his part, Mr. Abbott has recently highlighted his efforts to restrict how race and gender are taught in schools, an apparent nod to the unexpected success for Republicans in Virginia, where the governor-elect, Glenn Youngkin, won the governor’s race with a similar focus on schools. Mr. Abbott has also regularly drawn attention to his push to expand the presence of law enforcement and state National Guard troops on the border.

The campaign was likely to play out on two terrains in the state: its rapidly expanding suburbs, particularly on the outskirts of Austin, Houston and Dallas, and along the border with Mexico.

Mr. O’Rourke’s campaign planned aggressive on-the-ground organizing, going door-to-door in places like South Texas where a lack of in-person campaigning appeared to have hurt Democrats in 2020. Mr. O’Rourke was expected after his announcement to make his first campaign swing through the South Texas border region.