Sensing an opportunity for an upset victory, Marco Rubio spent most of Friday in Kansas, where he picked up a series of high-profile endorsements that he hoped could help thrust him into contention.

Instead, he finished a dis­appointing third in the Saturday caucus in Kansas, repeating the same pattern as in some Super Tuesday states earlier last week: a big last-minute push, notable endorsements and a thud of a finish.

Those doing the endorsing, along with many other supporters, bemoaned the results, as well as the campaign that produced them.

“I felt I had a dog in the fight, and it hurt me personally when I thought we were going to win,” said Republican Sen. James M. Inhofe of Oklahoma, another state where Rubio came in behind Cruz and Trump. “The thing is, when Rubio was there, the enthusiasm was so great, better than the others. He had a great reception. If everything had been equal in terms of appearances and organization, he would have won Oklahoma.”

Party leaders, donors and other supporters of Rubio portray a political operation that continues to come up short in its message, in its attention to the fundamentals of campaigning and in its use of a promising politician. The failures have all but doomed ­Rubio’s chances of securing the GOP nomination, leaving him far behind Trump and Cruz in both delegates and states won.

Many Rubio backers say they still believe Trump would be a political disaster but are worried that the freshman senator is not doing enough to make an effective case against the billionaire. Even with a strong win Sunday in Puerto Rico, Rubio has lost 18 of 20 nominating contests so far, and he faces grim odds in many of the states to come.

All of Rubio’s hopes now ride on his ability to win his home state’s 99 delegates. But even if he prevails in Florida’s winner-take-all contest, it will be difficult for him to secure enough delegates before the party convention in July, meaning he would have to try to win the nomination in an unpredictable floor fight.

“They have no infrastructure,” said Scott Reed, who is unaffiliated with any campaign but serves as the chief political strategist for the U.S. Chamber of Commerce. “His campaign hasn’t been able to keep up with his candidacy. . . . They don’t have the operation in the states to help him get over the top. He should be a finalist going all the way to California, and he’s not.”

One prominent Rubio supporter, who is an elected official and spoke on the condition of anonymity to comment candidly, said Sunday that he now doubts that Rubio can win and is privately preparing to support Cruz should the race narrow. He said Rubio’s recent decision to go hard in attacking Trump does not seem to have helped him.

“Cruz seems to be the only one benefiting from Marco’s hits on Trump. Marco hits Trump and doesn’t go up in the polls,” the supporter said. “It wasn’t easy to watch television on Saturday night. Nothing is happening. A lot of us, we love him, but we think we might have to end up with Cruz.”

One major Rubio donor was more dour, referring to Rubio’s dismal 2-for-20 record: “That’s not good.”

Sen. Marco Rubio (R-Fla.) speaks to supporters at Patrick Henry College in Purcellville, Va. on Feb. 28. (Ricky Carioti/The Washington Post)

The donor, who spoke on the condition of anonymity to talk frankly about strategy, said that despite his best efforts to reap fresh financial support for Rubio, “people are holding off. They’re not inclined to Trump or Cruz and are desperately seeking another alternative. But Rubio needs to show strength for that interest in him to be sustained.”

In recent days, Rubio has described the GOP race as “a protracted deal” because the contest remains crowded with Trump, Cruz and Ohio Gov. John Kasich.

“This map only gets better for us as we move forward in some of the other states,” Rubio said while campaigning in Puerto Rico on Saturday.

Rubio is scheduled to campaign Monday along the ­Interstate 4 corridor, which stretches from Tampa east to Orlando and is packed with swing voters. He is also expected to focus on his native South Florida — packed with a high percentage of Republicans who, like the senator, are of Cuban descent.

The race has tightened in Florida, according to recent polls, but Rubio still trails Trump. With just $5.1 million in his campaign account at the beginning of February, Rubio is relying on super PACs to air millions of dollars in attack ads against the front-runner. One of the state’s most revered Republicans, Jeb Bush, has so far declined to endorse him.

There is disagreement among Rubio’s supporters over whether his decision to aggressively attack Trump — belittling his character, his appearance and even his manhood — has worked. Rubio’s advisers felt he had to mock Trump to gain national exposure amid the round-the-clock attention given to the front-runner. But others fret that the attacks have damaged Rubio’s image as a young, optimistic party leader.

“We would be more than happy to check the insults at the door and focus on policy and focus on each candidate’s vision for the future of the country, but if the price we have to pay to get the media to cover the substance of our campaign is to mix it up a little bit, then we’re not opposed to doing that,” said Todd Harris, a senior Rubio adviser.

John “Mac” Stipanovich, a Tallahassee-based lobbyist who supports the senator, agreed with Harris: “The new Marco is much different, much feistier and in some peoples’ opinion much less dignified than the old Marco. But the old Marco was certainly going to lose. The new Marco may not win, but he has a much better chance than being so passive.”

But Sen. Tim Scott (R-S.C.), whose early support provided a crucial boost in South Carolina, said he supports focusing attacks on Trump’s many policy positions rather than on personal insults.

“I think it’s important to stay on topic and stay on the policy contrasts and differences,” Scott said last week.

Anthony Gioia, a top GOP fundraiser in Buffalo who served as ambassador to Malta during George W. Bush’s administration, said he is hopeful that Rubio’s more aggressive focus on Trump’s business history will alter the dynamics of the race.

“I just don’t think the substantive criticisms of Trump have really come out as much as they should,” he said, adding later: “I understand the role the president plays in the world. You can’t be a showman.”

Another concern among supporters centers on Rubio’s outreach to everyday voters and to big-name supporters.

As more than two dozen senior GOP Senate aides looked on Thursday afternoon at a private lunch presentation about the 2016 presidential race, conservative scholar Henry Olsen turned to Rubio’s chief of staff, Alberto Martinez, and offered a candid assessment of the senator’s campaign pitch.

Olsen said that while he appreciates Rubio’s sweeping call for building a “21st-century economy,” he worries that speaking in such broad terms in a year of populist unrest may be a political mistake.

Later, Olsen gently suggested that Rubio might be better served by fine-tuning his message with a more visceral appeal to working-class Americans who feel left behind and say he’s got their backs.

Martinez listened politely, and the conversation moved on.

The lunch, which was recounted to The Washington Post by three people present, was held at the Capitol. The regular sessions are deemed strictly off the record and allow Senate GOP chiefs of staff to hear from guest speakers about the political scene. The Rubio campaign and Olsen declined to comment.

According to senior congressional Republican aides, Rubio’s camp has told prospective endorsers that building momentum in the final week of that particular state’s primary or caucus was central to its strategy. The best illustration of this plan came in South Carolina, where Rubio had a strong performance in a debate just days ahead of the primary, held several well-received rallies and, finally, won the endorsement of the popular governor, Nikki Haley (R).

It brought him a better-than-expected finish, about 1,000 votes ahead of Cruz for second place.

Rubio made calls that night to senators in key states with primaries in the coming weeks, according to senior Senate aides familiar with the candidate’s calls. He secured the instant endorsement of Sen. Dean Heller (R-Nev.), whose support of Jeb Bush was freed up by the former Florida governor’s withdrawal after South Carolina. In Nevada, Rubio edged Cruz for second place.

The senator’s team thought it could keep replicating that effort — an impressive debate performance, packed rallies and a late endorsement from a popular figure. In the days leading up to Super Tuesday, according to a senior Senate aide, Rubio told prospective endorsers two key data points: that even after Trump’s expected delegate haul through March 15, the businessman would still be only about one-third of the way to securing the nomination and that Rubio would be clearly in second place, well ahead of Cruz in the delegate race.

That argument secured the backing of Sen. Lamar Alexander and Gov. Bill Haslam, two major figures in Tennessee’s Republican establishment just days before their state’s March 1 race. Also on board was Sen. Thom Tillis (R-N.C.), a powerful former state legislative leader whose state also will hold a primary on March 15.

But Rubio’s game plan ran into reality — the #MarcoMentum strategy, as it’s been dubbed on social media, was covering up massive deficiencies inside the states that were voting. Rubio had little to no infrastructure inside those key states, and each effort began when he was so far behind that momentum meant very little. He ended up a distant third behind Cruz, whose campaign has run a more effective, traditional effort to find supporters and then get them to the polls.

In pitches to fellow Republican senators, Rubio’s team highlighted its repeated support among “late deciders,” voters who make up their minds in the final week. But that meant little if the overwhelming majority of voters made up their minds earlier than that.

In Virginia, for example, Rubio won among late deciders, winning 39 percent of those making up their mind in the past week, twice as much support as Trump picked up that week, according to exit polls. But that bloc represented just 35 percent of the electorate — Trump won 42 percent of voters who decided before the last week of the campaign, enough to win.

Overall, Rubio finished third in eight of the 11 states last Tuesday, sometimes well behind Trump and Cruz, and won only in Minnesota. Cruz won three states and had five second-place finishes. Rubio placed third in Tennessee, despite Alexander’s and Haslam’s support. It was the same in Oklahoma, where he had won the endorsement of former senator Tom Coburn and held large rallies in Tulsa and Oklahoma City.

The pattern repeated Saturday in Kansas, where he had won support from Gov. Sam Brownback, former senator Robert J. Dole, Sen. Pat Roberts and Rep. Mike Pompeo — uniting the state’s often-feuding establishment and conservative wings.

Pompeo spoke on Rubio’s behalf at a caucus site Saturday in Wichita, where both Trump and Cruz appeared in person. Rubio was at the Conservative Political Action Conference (CPAC), a gathering of conservatives in suburban Maryland.

“Goodness knows how much of a difference it would have made” if Rubio had campaigned earlier and harder in the state, Pompeo said. “I do think had Senator Rubio had a chance to get his message out in a way that he wasn’t able to, he would have done better and would have competed much more closely.”

Meanwhile, Rubio supporters are angry and bitter about the continued presence of Kasich, whom they view as more of a stubborn spoiler than a legitimate contender for the presidential nomination.

On conference calls with donors and other supporters last week, Rubio campaign manager Terry Sullivan suggested that Kasich was effectively helping the Trump cause by pulling votes away from Rubio, according to those on the calls. Sullivan and other participants discussed ways to convince friends of Kasich to directly urge him to drop out, according to a participant on one of the calls.

But Kasich has rejected the overtures, and his campaign believes that states voting after March 15 in the Mid-Atlantic and West will be more favorable to him than Cruz or Rubio.

Looking ahead to Florida, ­Rubio’s team sees tightening polls and believes that a full week of campaigning in the state — plus another strong debate performance on Thursday night — should close the gap.

North Carolina retail executive Art Pope, a major Republican fundraiser supporting Rubio, said he has not seen any faltering among donors, adding that he believes there is a long-term path for him to prevail.

“Winning Florida is important, but it is not the beginning or end of the campaign, any more than Iowa was,” Pope said, adding: “If he wins Florida, it’s wonderful. If he loses Florida, the fight continues on to the convention.”

Matea Gold, Tom Hamburger, Philip Rucker and Sean Sullivan contributed to this report.