Former Colorado governor John Hickenlooper shown July 30 at the Democratic presidential primary debate hosted by CNN in Detroit, is polling among the lowest tier of candidates. (Carlos Osorio/AP)

For months, former Colorado governor John Hickenlooper has been under pressure from fellow Democrats to abandon his long-shot presidential bid and tackle a task closer to home — challenging vulnerable Republican Sen. Cory Gardner.

But as Hickenlooper’s White House aspirations sputter, there are signs that claiming a Senate consolation prize won’t be easy: The Democrats already running are giving no indication they intend to end their campaigns, meaning Hickenlooper could face a difficult primary featuring many of the troublesome currents he has struggled to navigate in the presidential race.

“This field is unlikely to stand down for him,” said Mike Stratton, a veteran Democratic consultant in Colorado who said Hickenlooper remained a formidable statewide candidate but that he could have trouble clearing the Senate field should he choose to run. “If you can’t get this to galvanize for you in the presidential, can you get it to galvanize for you in the primary in Colorado?”

For Democrats, Colorado is a must-win if they are to have any hope of the net gain of four seats they would need to retake the Senate majority — and thus make any Democratic president’s legislative agenda even close to viable.

Senate Minority Leader Charles E. Schumer (D-N.Y.) has kept a close eye on Hickenlooper’s campaign and made clear that he would be welcomed to the Senate fold, according to Democrats familiar with his thinking, which has been guided by private polling showing Hickenlooper easily winning the primary and beating Gardner.

But while Hickenlooper has spent months traveling in Iowa and other early-primary states, several candidates with strong profiles have built credible campaigns to take on Gardner.

Three candidates — former state senator Mike Johnston, former State Department official Dan Baer and ex-state House speaker Andrew Romanoff — have already raised more than $1 million. Former U.S. attorney John Walsh is not far behind that benchmark. Alice Madden, a well-respected former state House majority leader, recently joined the race, while Secretary of State Jena Griswold has launched an exploratory effort.

All are younger than the 67-year-old Hickenlooper and are well-positioned to outflank him by reminding Democratic primary voters about the parts of his centrist gubernatorial record that might be especially distasteful to liberals — including his strong support of the state’s oil and gas industry and his uneven support for gun-control measures.

While Hickenlooper’s brand of quirky moderation won him two terms as governor, he never had to face a competitive statewide primary, and his performance in the presidential race has only amplified questions about his viability in the state.

“Every day that he continues to run for president, he loses a little bit more luster as a potential Senate candidate,” said Dick Wadhams, a former Colorado Republican Party chairman and observer of state politics.

It remains unclear whether Hickenlooper is inclined to either abandon his presidential race or, if he does, switch to a Senate run. He could declare his Senate candidacy as late as March, though Colorado has a rigorous ballot qualification process that would make it difficult to wait more than a few weeks past the start of the new year.

He appeared to crack the door open to a Senate run in a Sirius­XM radio interview Sunday, saying he would “be a fool” to continue his presidential campaign if he remains mired under 2 percent support and that he’s “never ruled out anything” regarding a jump to the Senate race. In an interview earlier this year he called Schumer “one of the more persuasive people in my acquaintance.”

Spokesman Peter Cunningham declined to comment on how Hickenlooper might fare in the Senate race: “At this moment, there is no decision to enter.”

Should he do so, Hickenlooper faces a millstone of his own making: his own dismal words about serving in the Senate, which are likely to fuel arguments that voters ought to prefer a candidate who is more interested in the job at stake.

“I’m not cut out to be a senator,” he told Politico in February. “I’m a doer. That’s what gives me joy.”

As recently as last month, he said of the Senate, “I don’t think that’s my calling,” and he quipped at a June 13 National Press Club event, “If the Senate is so good, how come all those senators are trying to get out?”

He again suggested in the SiriusXM interview that he was less than enthused about the legislative lifestyle: “What excites me is being in those executive positions,” he said.

Four Democrats, in interviews or statements to The Washington Post, declined to criticize Hickenlooper directly but made implicit cases for their candidacies that highlighted his weaknesses.

Johnston — who has raised a staggering $3.4 million this year, outpacing Hickenlooper’s presidential fundraising — suggested in an interview that voters aren’t interested in a candidate who isn’t fully invested in the job.

“They’re tired of sending people to D.C. and not seeing any changes,” he said. “They know I am going to give every single ounce of what I have to this and that I am all in for fighting with everything we have to actually make change in the U.S. Senate and to be there for as long as it takes to do that.”

Romanoff, in a statement, pointed to his strong small-donor fundraising in promoting his candidacy. Hickenlooper has struggled to build support among small donors, and he is well short of the 130,000-donor threshold to qualify for the next round of presidential debates in September.

“Our grass-roots campaign has earned support from more Coloradans — including nearly 300 elected officials — than any other candidate, and I’d be glad to earn Governor Hickenlooper’s endorsement as well,” Romanoff said.

Baer identified in an interview with the crop of “national security Democrats” with backgrounds in foreign affairs, intelligence or the military running their first political race who were able to unseat Republicans in last year’s midterms. He served briefly as director of the Colorado Department of Higher Education in the final months of Hickenlooper’s tenure, has gotten donations from Hickenlooper’s wife and numerous members of his state administration, and was careful to note he meant no criticism.

“The best way to replace a career politician like Cory Gardner is with somebody who isn’t one,” Baer said, adding: “The person who will succeed in beating Cory Gardner is the person who is able to make this election at least in part about the fact that Cory Gardner has sold out again and again for Donald Trump and is able to not let him run away from that record.”

Hickenlooper has been famously averse to negative campaigning, from his first gubernatorial campaign in which he made it a centerpiece of his appeal to voters, to his presidential run, in which the closest he has gotten to an attack line is declaring that “socialism is not the answer.”

Walsh is running on his six years’ experience as Colorado’s top federal prosecutor under President Barack Obama. In that role, he helped manage investigations into mass shootings in an Aurora movie theater and a Colorado Springs Planned Parenthood clinic and national probes into Wall Street and the pharmaceutical industry that led to multi­billion-dollar settlements.

His approach to questions about Hickenlooper is to simply cast doubt that the former governor will ultimately enter the race: “He’s said over and over again that the Senate is not the kind of position he wants, and for that reason, I think it’s pretty clear that his mind is focused on other things,” Walsh said, adding: “I’m 100 percent committed to seeking this office, and I’m trying to make sure everybody knows that.”

Multiple Democrats with experience in Colorado politics, meanwhile, are warning not to take the state for granted. They note that Gardner, 44, is a talented campaigner with an appealing profile who won what many considered to be an impossible race in 2014, beating incumbent Democrat Mark Udall by a razor-thin margin.

Gardner spokesman Jerrod Dobkin declined to comment on any particular opponent but said the Democratic field “is full of candidates embracing radical positions” on health care, immigration and other issues.

“The eventual nominee will stand in sharp contrast to Senator Gardner, who . . . has a record of getting things done for Colorado,” he said.

Still, Democrats maintain that Gardner, who has broken with President Trump at times but has been a reliable vote for key administration agenda items, will have a tough path to reelection running alongside Trump in a state the president lost by five percentage points in 2016 — whether Hickenlooper runs or not.

“It’s a wide-open field, but it’s a talented field,” said Craig Hughes, a consultant with ties to Hickenlooper and several of the state’s most prominent Democrats. “The national dynamics are working against Gardner, and that as much as anything puts him in a very difficult position, whether it’s standing with Trump on immigration or health care or the Trump tax cuts. It makes him incredibly vulnerable to any number of Democratic candidates.”

Holly Bailey and David Weigel contributed to this report.