Top donors warn that Republican presidential candidate Jeb Bush needs to quickly turn things around or risk defections. (Charlie Neibergall/AP)

Jeb Bush is entering a critical phase of his Republican presidential campaign, with top donors warning that the former Florida governor needs to demonstrate growth in the polls over the next month or face serious defections among supporters.

The warnings, expressed by numerous senior GOP fund­raisers in recent days, come as Bush and an allied super PAC are in the early stages of an aggressive television ad campaign they say will help erase doubts about his viability.

But Bush continues to battle against a steady decline in the polls, sinking to fifth place at just 7 percent in a national NBC News/Wall Street Journal poll released Sunday and similarly languishing in the early-voting states of Iowa and New Hampshire.

The warnings from top donors come as Wisconsin Gov. Scott Walker’s exit from the race re­focused the battle within the GOP’s establishment wing as one between Bush and his former protege, Sen. Marco Rubio (R-Fla.). Right now, the momentum appears to be behind Rubio, who has jumped ahead of Bush in most polls. At least a third of the bundlers who signed up to raise money for Walker have switched their allegiance to Rubio, while a smaller number have gone with Bush, according to people familiar with the discussions.

Bush also is facing fresh scrutiny for comments that critics say bear echoes of remarks Mitt Romney made during his 2012 GOP presidential bid, part of a pattern of awkward statements that Bush or his campaign have had to clarify.

Campaigning in South Carolina last week, he said that Democrats often win over black voters by telling them “we’ll take care of you with free stuff.” Romney made similar comments during his 2012 presidential bid. Democrats said that Bush’s remarks were part of a pattern of Republicans insulting minority voters.

His comments could undercut what Bush allies argue would be his great strength in a general-election contest: a cultural fluency that would give him crossover appeal with a diverse electorate.

Party strategists said that Bush must find a way to recharge his campaign with a compelling message about his conservative governing record in Florida.

“People assume that they know who Jeb Bush is, and that’s part of the struggle the Bush campaign has,” Republican strategist Henry Barbour said.

“I think if people get to know Jeb and they give him a chance, he’s going to be tough to beat,” Barbour added. “But they don’t know him yet. And you’ve got a right wing of the party that is almost determined not to get to know him. They want to believe that because they disagree with him on a couple issues that he’s not their guy.”

Appearing on “Fox News Sunday,” Bush played down the importance of his standing in polls. “These polls really don’t matter. . . . They don’t filter out the people that aren’t going to vote, it’s just — I know it’s an obsession because it kind of frames the debate for people for that week,” he said.

Sen. Marco Rubio (R-Fla.), left, is now polling ahead of former Florida Gov. Jeb Bush in the race for the 2016 Republican presidential nomination. (Left: AP/Carolyn Kaster)

As the son and brother of former presidents, Bush is a member of the Republicans’ establishment wing, which has suffered setbacks in recent days. House Speaker John A. Boehner (R-Ohio) — who publicly encouraged Bush to enter the race — announced plans to resign Friday after dozens of conservative lawmakers threatened to try to oust him. And Walker’s sudden departure from the presidential race came as support shifted to candidates who are seen as outsiders.

Given those developments, Bush “needs to get his favorables up,” said a senior GOP bundler who is backing him and requested anonymity to speak candidly.

“People are looking at the stage and saying: ‘Jeb and Marco? I’m going with the new,’ ” said a top party fundraiser not aligned with a campaign. “You’re seeing people really gravitate to [Rubio] and saying, ‘Okay, we’ll buck the Bush machine.’

“What I hear everywhere when you say Jeb’s name is, ‘If you want to lose the general election, nominate Jeb,’ ” the fundraiser added.

But those within the Bush camp say they are not alarmed over the dynamics of the race, confident that their financial war chest will enable their candidate to outlast opponents, according to campaign strategists and top Republicans familiar with internal discussions.

In private conversations, however, Bush advisers betray signs of anxiety about Rubio’s rise.

“They are prepared for a long, grinding fight and being the last person standing,” the GOP fundraiser said. “But they are concerned about the trends, and they are concerned about Marco.”

Rubio has jumped ahead of Bush in recent polls of Republicans in New Hampshire and Florida — two states critical to Bush’s campaign strategy. In New Hampshire, a CNN/WMUR poll released last week gave Trump a commanding lead with 26 percent. Rubio was third with 9 percent, and Bush tied with Ohio Gov. John Kasich for fifth place, with 7 percent. A Florida Atlantic University poll released last week put Trump’s support among Sunshine State Republican voters at 31.5 percent, with Rubio at 19.2 percent and Bush at 11.3 percent.

Bush maintains a huge financial advantage over Rubio — Bush had about $120 million between his campaign and an allied super PAC at the end of June. To keep his edge, he has recruited his family to help. Former president George W. Bush is headlining a series of fundraisers, an effort that began with a Sept. 10 luncheon in New York. His wife, Laura, and Jeb’s wife, Columba, and his son Jeb Jr. also are taking part in finance events.

The candidate kept a breakneck pace as he traveled the country raising money in the third quarter, holding events in New York, Los Angeles, Miami, Boston, Seattle, Las Vegas, Salt Lake City, California’s Silicon Valley, Portland, Ore., and Birmingham, Ala. As the quarter draws to a close, he is scheduled to raise money in St. Louis on Monday and Oklahoma City on Tuesday morning.

There is a big prize being dangled to motivate fundraisers: Those who help bring in at least $50,000 by Wednesday will be invited to a “Jeb Celebration” retreat in Houston in late October that is to be attended by three generations of the Bush family, including both former presidents.

With his financial dominance, Barbour said, Jeb Bush “is clearly a guy who has built a campaign that is looking beyond the first few states and is in it for the long haul.”

“I think the race is fairly wide open and there are a lot of people with a path to the nomination. Some more than others,” Barbour said. “I think Jeb’s is better than most. But he’s got to continue to perform well and arguably better.”

One persistent problem for Bush has been a tendency to wander into verbal culs-de-sac, drawing the ire of minorities, women, Democratic leaders or Republicans.

Over the summer, Bush suggested that the federal government should stop funding women’s health programs. He later said he misspoke. He defended the term “anchor babies” — widely viewed by Asians and Hispanics as offensive — as an appropriate way to describe how some undocumented immigrants exploit U.S. citizenship laws. And after saying that “people need to work longer hours,” he clarified that he meant Americans need more full-time rather than part-time work.

Democrats warn they will use Bush’s tongue-twisted answers against him if he becomes the GOP nominee.

“Jeb Bush either has no idea what he’s talking about or he’s a cynical politician appealing to the ugliest elements of the Republican Party,” said Michael ­Tyler, spokesman for the Democratic National Committee. “Either way, he is unfit to lead this country.”

Dan Pfeiffer, a former senior aide to President Obama, said last week on Twitter that given Bush’s most recent comments on black voters, “If Jeb’s last name was Walker, he would be out of the race by now.”

Bush said in the Fox appearance that his remarks were taken out of context by Democrats. “I think we need to make our case to — to African American voters and all voters that an aspirational message, fixing a few big complex things, will allow people to rise up,” he said. “That’s what people want. They don’t want free stuff. That was my whole point.”

Before the Sept. 16 Republican candidates’ debate at the Reagan Presidential Library, senior Bush strategists believed that he needed to improve his performance to stop a precipitous drop in polling that began shortly after Donald Trump entered the race. A more pugnacious performance — including an off-the-cuff defense of his brother prompted by an attack by Trump — was cheered by some supporters as a sign of much-needed passion.

“As people start to focus and look at each person and the positions they have on how to take the country forward, I think Jeb will do well,” said Bill Kunkler, a Chicago private equity executive who is helping Bush raise money. “Right now the polls are measuring mood. It’s more like picking a date than a mate.”