Charles Schumer, Marco Rubio,  John McCain

Authors of the newly released Senate immigration bill touted the package Thursday as a “bipartisan breakthrough” in advance of a critical hearing, as opponents began to organize against the bill — claiming it doesn’t do enough to enforce existing immigration law.

Sen. Marco Rubio, R-Fla., who has put his conservative reputation on the line with his involvement in writing the bill, took to the floor late Thursday afternoon to defend it. Though critics have homed in on the bill’s pathway to citizenship for illegal immigrants, Rubio said the package would also fix a “broken” legal immigration system so that foreign students trained in America would not be sent back home once they’ve learned their skills.

“If there wasn’t a single illegal immigrant in the United States, we would still have to do immigration reform,” Rubio said.

As for the path to citizenship, which would give up to 11 million illegal immigrants a shot at legal status, Rubio said “the alternative is to do nothing” — which he described as “amnesty.”

Rubio and the seven other co-authors, who formally unveiled the legislation at a press conference Thursday, are hoping to avoid the fate of the 2007 immigration bill, which died amid heated criticism from both sides of the aisle. Republicans have bluntly professed an interest in courting Hispanic voters, and some prominent members, including Rubio, have lent their name to the effort.

But critics were building a multi-faceted case against the bill, in advance of Friday’s hearing on the Senate Judiciary Committee. They claimed the bill grants “amnesty” without enough enforcement provisions. They claimed it would burden taxpayers by eventually plugging legalized immigrants into the public welfare system — a claim Rubio has strongly challenged. And Republican critics joined immigration enforcement officers in claiming the bill would not address a major loophole — giving the government “discretion” to choose when to enforce immigration laws.