Sen. Marco Rubio (R-Fla.), left, is battling for many of the same voters as former Florida governor Jeb Bush, right. (Left: Carolyn Kaster; Right: Pablo Martinez Monsivais/Associated Press)

Amid the Jeb Bush buttons and appearances by three generations of his family, there was a subtle but unmistakable focus on another candidate for president at a two-day donor summit here: Sen. Marco Rubio.

In a closed-door strategy briefing, Bush campaign officials detailed numerous contrasts they are seeking to draw with Rubio (R-Fla.) and branded him disparagingly as a “GOP Obama.” An official with a pro-Bush super PAC mentioned Rubio’s name twice in a chat with reporters, despite not being asked about him. And a Bush ally suggested that Rubio should think about resigning from the Senate given his focus on the presidency.

While Donald Trump has seriously altered the course of Bush’s campaign, it is clearer than ever that Bush ultimately sees Rubio, who is rising in the polls, as the biggest obstacle in the lane he is trying to take to the GOP nomination.

“The higher you climb on the ladder, the more behind you show and the bigger target you become,” said former Republican National Committee chairman Michael S. Steele, who is neutral in the primary race.

Updating top donors at a hotel ballroom here Monday, Bush’s aides compared his many Florida endorsements with Rubio’s more modest support among Sunshine State Republicans, according to highlights of the presentation shared by the Bush campaign. They also likened the young senator to President Obama, arguing that the two have “strikingly similar profiles” in their relative lack of executive experience while seeking the White House.

Jeb Bush’s campaign is struggling, recently cutting payroll and staff. (David Becker/Getty Images)

Speaking to reporters on the sidelines of the summit Sunday, an official with Right to Rise USA, the pro-Bush super PAC, criticized Rubio twice over fundraising. Meanwhile, Rubio’s many missed Senate votes have drawn criticism from Bush and others. Former senator George S. LeMieux, a Florida Republican who attended the summit, said Rubio ought to consider resigning from Congress as he pursues the presidency.

Even Bush took a dig at Rubio, without mentioning him by name.

“I vetoed a couple of projects for one of the presidential candidates,” he said during a moderated discussion. Bush served as governor of Florida when Rubio was in the state House.

Rubio’s campaign did not immediately respond to the criticism. The 44-year-old first-term senator has rebutted the idea that he is a Republican version of Obama, saying in Iowa over the summer that Obama was a “backbencher” in the Illinois legislature while he was the Florida House speaker. He also noted that he has served a couple of years longer in the Senate than Obama had when he first ran for president.

The meeting of Bush donors and family loyalists in downtown Houston on Sunday and Monday came at a crossroads for Bush’s campaign. Lagging in early-state surveys amid the rise of political newcomers such as Trump and struggling to keep on pace with fundraising, the campaign took dramatic steps last week to cut costs, trimming payroll and headquarters staff.

The slimmed-down operation means that more responsibility to organize may fall to Right to Rise, which has so far focused on TV ads touting Bush’s record.

Marco Rubio is rising in the polls. (David Becker/Reuters)

“Yeah, we’re looking at some of that,” the Right to Rise official said of the possible deployment of ground staff to key states. “The campaign is front and center on that, but there are a lot of supporters around the country who might want to be organized to do some stuff.” The official spoke on the condition of anonymity to discuss strategy freely.

Donors here said they were mostly upbeat about Bush’s chances, despite his struggles. They perused the Bush swag and mingled with former presidents George W. Bush and George H.W. Bush. The latter wore a “Jeb!” button to a barbecue dinner Sunday night.

“You can’t run against Ben Carson and Donald Trump,” said Fred Zeidman, a fundraising bundler for Jeb Bush. “They are not campaigning on issues. They are campaigning on emotional attachment.”

But Rubio could be a bigger threat in the long run. He is competing for many GOP establishment donors whom Bush wants or already has in his corner. As Bush has declined in the polls, many Rubio allies are eager to poach his financial support.

“One longtime Bush donor told me: Make sure there is room for me there when this thing goes down,” said a bundler connected to Rubio’s campaign who spoke on the condition of anonymity to describe private discussions.

Bush will embark on a busy campaign schedule in the early-voting states after Wednesday’s CNBC debate in Boulder, Colo. His backers are hopeful that his electability will help him break through in a way it hasn’t so far.

Theresa Kostrzewa, a Republican fundraiser from North Carolina who attended the gathering in Houston, said she was impressed with new TV spots screened by the campaign and the super PAC and more convinced than ever that Bush is the right choice. “We’re trying to elect a president, not someone who is going to be great on Jimmy Fallon,” she said.

“It’s like a lot of dating,” Kostrzewa added. “The voters are out there speed-dating, but they haven’t really gotten serious. And Jeb Bush is the marrying kind.”

During the moderated discussion Monday afternoon, George W. Bush highlighted his brother’s appeal to Hispanic voters as evidence that he is the most electable candidate. On that selling point, however, he will face competition from Rubio, who would be the nation’s first Hispanic president.

“Jeb is going to win the Latino vote, which is essential to winning,” George W. Bush said. “This campaign ought to be about who can win the White House.”

Matea Gold and Tom Hamburger in Washington contributed to this report.