Sen. Marco Rubio, R-Fla., is in the midst of a last-minute push to garner support for the Gang of Eight bill from conservative critics, but the window to do so may be closing, Republican aides say, in part because of how Senate Majority Leader Harry Reid, D-Nev., is managing the process.

Rubio said last week that he was with GOP colleagues to strengthen the border security language in the immigration bill. “I don’t want to leave the border plan up to the Department of Homeland Security and Janet Napolitano,” Rubio told Ed Morrissey during an interview on the Hugh Hewitt Show last week. “That is a mistake that is in the current bill. I think what we need to do is detail minimum standards, at a minimum what the border plan should be, and detail it. It must have this.”

Earlier border security amendments have been denounced as poison pills by Rubio’s allies on the Gang and among Senate Democrats at large, including a proposal from Sen. John Cornyn, R-Texas — that might not have won many Republican votes — and from Sen. Chuck Grassley, R-Iowa., which had more support in the GOP conference.

“The fact is, Senator Rubio is bouncing around trying to find a path out,” a Republican staffer told the Washington Examiner. “The bill is indefensible — and he has all but admitted it by saying ‘it must be improved.’  So he saddled up with Cornyn to try to get cover — but now that doesn’t seem to be working either. Now there are more secret back room-deals being attempted — while Harry Reid and Chuck Schumer stiff-arm Senator Grassley and other Republicans who are trying to modify the bill on the floor.  The American people are just waking up to the reality of another 1000 page amnesty bill — and there is a long way to go in this debate.”

Rubio stayed optimistic about gathering Republican support. “I think you’ll see something, God willing, early next week so people can start to look at it. A bunch of senators have been working on it,” he told Morrissey. “A lot of Republicans want to be supportive of something, but need to be able to go back home and tell people that they have taken serious steps to make sure this never happens again.”

The idea of having to back a substantial change to the bill on such short notice.”How long have they been working on this bill?” another Republican Senate  aide said. “And they are going to surprise everyone with a brand new bill just DAYS before senators vote on it, before anyone can read it, score it, evaluate it?”

“I have not heard those complaints,” Rubio spokesman Alex Conant replied when asked about it. “We are working with our Republican colleagues to improve the legislation.”

A staffer in a fourth Senate office (not Rubio’s) cautioned against criticizing the Florida lawmaker about the timing. “Harry Reid’s the one who is responsible for giving us enough time to debate something,” the aide told the Washington Examiner.

Reid, though, has announced that he is determined to pass the bill before the July 4th recess, probably in part because of the danger that public opinion will turn against the current form of the bill as opponents continue to attack it.

“There is no obvious, pressing reason for Reid’s schedule. But as he spoke, there were signs that popular support for immigration reform is slipping,” the Washington Examiner‘s Byron York pointed out last week. “The polls might not have much more time to change, if Reid goes through with his schedule and immigration reform gets a final vote by the end of next week.”

On scale, aides suggest that most Republicans won’t support the bill, as Reid and Sen. Chuck Schumer, D-N.Y., who leads the Democratic wing of the Gang of Eight, haven’t made any substantial compromises on border security.

“I think Harry Reid has been overly-confident about his about his ability to attract broad Republican support for a bill that needed improvements,” as one of the aides speculated.

At a minimum, though, Reid has succeeded in forcing Rubio — a Tea Party favorite and leading contender for the 2016 Republican nomination — to expend political capital with his base by trying to convince conservatives to support a bill that fails to meet the border security demands they’ve made in the past.