WASHINGTON — Sen. Marco Rubio has for months positioned himself as the focus of the immigration debate, the reason why a bill has gotten as far as it has. But now he has managed to create an aura of mystery: Is he on board or not?A series of increasingly mixed signals from the Florida Republican — including telling a conservative radio host Tuesday he would vote against the bill he helped write if changes aren't made — has frustrated and worried reform advocates as the Senate is scheduled to begin debate today.Which way Rubio, a potential 2016 presidential candidate, goes is crucial. House Republicans are already resisting the Senate approach."In politics, perception is everything, and it's deeply concerning a lot of us," said David Leopold, former president of the American Immigration Lawyers Association. "I have a lot of respect for Sen. Rubio, but a true leader puts politics aside. This is not an issue you can play both sides."Gaby Pacheco, an activist from Florida who has come to admire Rubio, said the comments marked the first time she has been worried. "I think he's trying to separate himself from the bill," she said. "If you do something, you just don't halfway abandon it. This bill is his child and when a child grows and there are issues, you don't disown him. You work through it. The bill was moving to the right anyway. I don't see why he has to say this."At the same time, Rubio has given hope to critics of the legislation that he will withdraw. "This is a very dramatic development," said a practically giddy Sen. Jeff Sessions, R-Ala.Rubio said he is simply pushing to strengthen the bill by adding more border security and enforcement measures. "One of the reasons why I was asked to even join this effort is to help bring Republicans on board. That's what I'm trying to do," he said.Asked pointedly in an interview Thursday if he was walking away, Rubio replied, "I'm 100 percent committed to immigration reform. But I'm committed to an immigration reform law, not an immigration reform bill. Somehow if efforts here were to slow down, for whatever reason, I'll keep working. I won't abandon this issue until it gets done."The 42-year-old son of Cuban immigrants is caught between strong forces: A diverse pro-reform coalition that includes Evangelicals, labor unions and business and Republicans eager to open doors to Hispanic voters versus vehement opposition from conservatives and anti-immigration interest groups. The Federation for American Immigration Reform, which wants fewer people coming in the country, said this week that it has started a TV ad in Tampa and elsewhere across the country that asserts Rubio is pushing the same failed promises of the 1986 amnesty bill.Rubio has done much to quiet conservative opposition but the push back has grown as the Senate bill has advanced. On the Laura Ingraham Show, where the host has blistered Rubio for weeks, a tea party leader from Colorado called in this week to say Rubio has "lost the tea party. There is no support for him on this issue." The equivocating is not new for Rubio.He took a hard-line approach on immigration as a U.S. Senate candidate in 2010 after being seen as a moderate in the Florida House. He opposed the Dream Act, which would create a path to citizenship for children of illegal immigrants, then proposed his own version to give them legal status. But Rubio never filed a bill and President Barack Obama stepped in to accomplish something similar. After joining the Senate Gang of 8 that wrote the current bill, Rubio emerged as an enthusiastic champion, doing a mind-blowing number of media interviews. But as conservative criticism has grown, he has gotten more pessimistic and sided with Republicans who say the bill he helped write is flawed."What's stymieing efforts in the Senate is not my comments," he insisted. "What's stymieing efforts in the Senate is that we don't have the votes to pass it because too many members on both sides of the aisle do not believe it goes far enough on border security."Advocates of reform say they understand Rubio's delicate dance — and think he's too far in now to back away — but fear the bill will move too far to the right."I can understand the value of being hard to pin down," said Ali Noorani, executive director of the National Immigration Forum. "But Sen. Rubio is at his best when he has a clear strategy on how he is going to build consensus. You can't build consensus in one place and then go around the corner and try to undermine it." Rubio's Gang of 8 colleagues do not sound too worried. "I think he's trying to grow the vote," said Sen. Lindsey Graham, R-S.C. Sen. Charles Schumer, D-N.Y., sounded as if he was working off Rubio talking points: "My view has always been that the American people will support common sense solutions to the 11 million and future legal immigration if they feel future illegal immigration will be stopped, and a strengthened border is a good part of that."One of the ideas Rubio is helping develop would shift the responsibility of developing a border security strategy from the Department of Homeland Security to Congress. Rubio said Thursday the idea is to insert the plan in the bill.Rubio also has worked with Sen. John Cornyn, R-Texas, who is proposing a sweeping amendment that would require 100 percent operational control of the U.S-Mexico border before undocumented immigrants can get permanent legal status. Cornyn would add more Border Patrol agents and exclude immigrants who have committed more serious misdemeanors from qualifying for legal status.Against wobbly signs, GOP strategist Karl Rove, took to the Wall Street Journal on Thursday with a bit of a warning:"As the Senate takes up immigration reform next week, Republicans must consider the impressions they will create by what they say, the changes they propose and their votes on the final product."