WASHINGTON -- Sen. Marco Rubio has, for months, positioned himself as the focus of the immigration debate, the reason why a reform bill has gotten as far as it has. But now he has managed to create an aura of mystery: Is he still on board with it or not?

A series of increasingly mixed signals from the Florida Republican — including telling a conservative radio host Tuesday that he would vote against the bill he helped write if changes are not made — has frustrated and worried reform advocates as the Senate is scheduled to begin debate Friday .

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 Sen. Jeff Sessions, R-Ala.

Rubio said he is 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 during an interview Thursday whether 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, business and Republicans eager to open doors to Hispanic voters versus opposition from some conservatives and anti-immigration interest groups.

The Federation for American Immigration Reform, which wants fewer people coming into the country, said this week that it had launched a television ad 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 pushback has grown as the Senate bill has advanced. On Laura Ingraham’s radio 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 then instituted something similar by executive order.