(CBS News) Backers of the bipartisan plan for comprehensive immigration reform face another challenge Thursday, as the Senate Judiciary Committee begins debate on the bill. The committee will consider the hundreds of amendments submitted by committee members this week, measures that range from protecting the U.S. economy to protecting LGBT immigrants and couples.
Senator Marco Rubio, R-Fla., who co-sponsored the bill, rejected claims that the proposed amendments amount to an effort to "gut" the bill.
"I don't think amendments are trying to gut it ... that's the way the process is supposed to work," Rubio said Thursday on "CBS This Morning."
"We worked with eight people -- four Republicans and four Democrats -- [and] crafted what I think is an excellent starting point and now we're asking our colleagues for suggestions on how to improve."
Rubio allowed that "some of those would have the effect of destroying the bill," but said, "I think that's true on both sides" and "the vast majority of amendments are actual efforts to improve it and we welcome that."
The Florida senator also took aim at a study recently released by conservative think tank, The Heritage Foundation. The report claims the Senate bill would cost the government up to $6.3 trillion over the next five decades to provide benefits for the estimated 11.5 million people now living illegal in the U.S.
"The study is not a legitimate study," Rubio said, explaining, "It overestimates the number of people that are going to be counted in the system ... it includes four million kids that are already living here now ... a quarter of the costs are for traffic and police and ... public services that are already being provided."
"The bottom line is, I think the study is deeply flawed," he insisted.
Rubio pointed out a letter released late Wednesday by a coalition of 24 conservative groups supporting legislative action on immigration reform.
"Basically every major conservative group out there that is involved in immigration is in favor of doing something akin to what we're putting together," Rubio said. The statement -- which was organized by the chairman of the American Conservative Union, Al Cardenas -- refers to the pending legislation as an "important starting point" and reads in part, "Conservatives are ready to support immigration reform, so long s it is pro-economic growth ... and prevents another wave of illegal immigration from happening again."
Rubio echoed that sentiment on Thursday, making the claim that the only conservative demand "is that the border is secured so this never happens again" and outlining the proposed plan to achieve bolder security."
"What we've said is 90 percent of the people who are trying to cross [the border] are apprehended ... we give them five years to reach that point and if that doesn't happen, then a border commission has $2 billion additional dollars to come in and finish the job."
"But it's not just a fence. It's also e-verify, it's also an entry-exit tracking system," Rubio said.