Sen. Marco Rubio Marco Antonio RubioHillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll Intelligence chief says Congress will get some in-person election security briefings Republican Senators raise concerns over Oracle-TikTok deal MORE (R-Fla.) is feeling the heat as potential GOP allies are now turning against his immigration reform bill.

Republican colleagues who were previously viewed as possible “yes” votes are keeping their distance from the 1,075-page measure.

On Wednesday, conservative activists ripped Rubio and Sen. John McCain John Sidney McCainKelly's lead widens to 10 points in Arizona Senate race: poll COVID response shows a way forward on private gun sale checks Trump pulls into must-win Arizona trailing in polls MORE (R-Ariz.) — both sponsors of the bipartisan immigration measure — at a Capitol Hill rally.

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The crowd, which was protesting the bill’s effort to put millions of illegal immigrants on a path to citizenship, booed at the mention of Rubio’s name during the six-hour event.

The rally underscored the mounting opposition to the immigration reform bill among both grassroots conservatives and GOP senators.

Potential Senate Republican backing for the legislation has begun to evaporate because of deep skepticism over its border-security and enforcement provisions.

In recent weeks, senators on both sides of the aisle speculated that 70 members of the upper chamber might vote for the Senate bill. That tally is now seen as unattainable.

Republican Sens. Mitch McConnell Addison (Mitch) Mitchell McConnellSenate Republicans signal openness to working with Biden Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll The Hill's Campaign Report: Biden asks if public can trust vaccine from Trump ahead of Election Day | Oklahoma health officials raised red flags before Trump rally MORE (Ky.), Rand Paul Randal (Rand) Howard PaulGOP senator to quarantine after coronavirus exposure The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill Overnight Health Care: Senate Democrats block GOP relief bill | Democrats reveal Medicaid chief's spending on high-paid consultants | Trump calls question about why he 'lied' about COVID-19 a 'disgrace' MORE (Ky.) and John Cornyn John CornynSenate Republicans signal openness to working with Biden Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll Republican Senators raise concerns over Oracle-TikTok deal MORE (Texas), who were once viewed as possible supporters of the bill, are now expected to reject it.

Rubio, a possible 2016 presidential candidate, is seen as essential to passing the measure, but his involvement hasn’t persuaded many of his GOP colleagues to back it.

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Many GOP senators say they want far more border-security provisions in the bill.

Republican Sens. Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (Tenn.) and John Hoeven John Henry HoevenDavis: The Hall of Shame for GOP senators who remain silent on Donald Trump Bottom line Bipartisan senators seek funding for pork producers forced to euthanize livestock MORE (N.D.) met Wednesday with members of the Gang of Eight, who drafted this bill, to negotiate a compromise on enforcement measures.

Corker told The Hill after the meeting that he and Hoeven thought there had been “substantial breakthroughs” with Democrats.

Corker and Hoeven declined to comment on the details of their proposal.

“We had a very positive morning, and we’re all now talking within our caucuses, and I’ll leave it at that,” Corker said.

Sen. Charles Schumer Chuck SchumerDemocrats scramble on COVID-19 relief amid division, Trump surprise Pelosi, Schumer 'encouraged' by Trump call for bigger coronavirus relief package Schumer, Sanders call for Senate panel to address election security MORE (D-N.Y.), who has strongly defended the border-security language in the bill, said, “I’m not talking about any specifics.”

Senate Republican sources briefed on the talks said Corker, Hoeven and the Gang of Eight are mulling an increase of 20,000 border patrol agents along the southern border. This would nearly double the number of agents along the U.S.-Mexico border.

But the Corker-Hoeven push for allocating thousands of additional border agents appeared to change few minds within the Republican conference.

“I didn’t see it move many people. It’s not going to get me to vote for it,” said a lawmaker who attended a meeting at which Republican senators discussed the proposal. “I’m not going to vote for an immigration bill that doesn’t have a metric for securing the border.”

There is now growing uncertainty about whether Rubio — in the face of sustained criticism on the right — will end up voting for the Gang of Eight’s bill.

Rubio has declined to outline his plans, unlike other members of the Gang of Eight, such as Sen. Lindsey Graham Lindsey Olin GrahamSenate Republicans signal openness to working with Biden Loeffler calls for hearing in wake of Netflix's 'Cuties' Quinnipiac poll shows Graham, Harrison tied in South Carolina Senate race MORE (R-S.C.), who have pledged their support.

Rubio ducked a question Sunday during an interview on ABC’s “This Week” about whether he would vote for final passage.

“I don’t want to get involved in hypotheticals and ultimatums,” he said.

He held to that line on Wednesday, telling a reporter, “I’ve already discussed it.”

Rubio has said he wants to see what kind of improvements will be made to the legislation.

In response to the proposal to dramatically boost the number of border patrol agents, he said, “We’ll have more to say about that later.”

Rubio has endured a spate of critical press reports in recent days.

The New Yorker’s Ryan Lizza reported that an unnamed Rubio aide told him in an interview that some American workers “can’t cut it” as a rationale for giving visas to foreign workers.

Conservative press outlets who hold significant sway in presidential races have also criticized Rubio. For example, the right-leaning National Review has dubbed the senator’s efforts as “Rubio’s folly.”

Meanwhile, Rubio has distanced himself from other members of the Gang of Eight in recent weeks.

He has opted out of public appearances with them and did not join in Wednesday’s meeting with Corker and Hoeven, according to a person who attended.

Rubio’s staff says he is still aggressively selling the bill to conservatives.

The Florida senator appeared Wednesday morning on a national radio talk show hosted by former Arkansas Gov. Mike Huckabee and was scheduled to give a television interview with Fox News host Sean Hannity on Wednesday evening.

Rubio remains popular in his home state, giving him leeway to take a gamble on pushing landmark legislation.

There are also signs the prominence he’s taken in pushing immigration reform is doing him some political harm.

A Quinnipiac University poll released Wednesday showed that Florida voters give him a 51 percent to 35 percent job approval rating.

Only 33 percent of all respondents said they approved of his handling of immigration reform, compared to 41 percent who disapproved.

Republicans, however, approved of his handling of immigration by a margin of 52 percent to 24 percent.

It appears increasingly unlikely the legislation will be able to meet the ambitious goal set by Schumer, McCain and Graham of passing with 70 votes.

Members of the Gang of Eight can now only be sure of 61 or 62 votes for final passage.

“Obviously, we want to get as large a vote as possible. We’re working on it. I think it’s clear there’s like 61 or 62 votes. What we want to do is grow it as much as possible,” McCain said.

McCain said he hoped the 70-vote goal was not overly ambitious.

The danger is that by setting such a lofty target at the outset, the bill may appear to be losing momentum if it barely clears the 60-vote threshold needed for passage.

Striking a deal with Corker and Hoeven on increasing border patrol agents by as many as 20,000 may not bring a flood of Republican votes, but at least it would secure the support of those two lawmakers.

That could, in turn, bring along Corker’s home-state colleague, Sen. Lamar Alexander Andrew (Lamar) Lamar AlexanderTrump health officials grilled over reports of politics in COVID-19 response Now is the time to renew our focus on students and their futures CDC says asymptomatic people don't need testing, draws criticism from experts MORE (R-Tenn.), a pragmatic deal-maker who leans toward the center.

Senate aides say if the Gang of Eight agrees to Corker’s and Hoeven’s demands to boost federal spending on border security, they will be expected to vote “yes.”

One critic of the bill called the Corker-Hoeven proposal a desperate “Hail Mary” to increase Republican support.

The Gang of Eight rejected a proposal by Cornyn to add 5,000 border patrol agents along the southern border during last month’s Judiciary Committee markup.