* U.S. senator criticizes legislation he helped write
* Toughening border security necessary for bill's passage - Rubio
By Caren Bohan
WASHINGTON, May 30 (Reuters) - Republican Senator Marco Rubio's repeated criticism of parts of the sweeping U.S. immigration bill he helped craft has unsettled immigration reform advocates and others who support its passage.
Rubio, a Cuban-American lawmaker from Florida seen as a possible 2016 presidential contender, has consistently defended the bill's centerpiece - providing legal status for roughly 11 million undocumented immigrants.
But he has publicly said he wants changes to the bill and said last week that he agreed with some of the concerns voiced in a letter by 150 prominent conservatives who oppose the legislation. He also told CNBC that it would not pass without changes.
His office also raised eyebrows a few weeks ago when it circulated a memo to other Senate staff listing 21 concerns with the bill and ideas for changing what would be the biggest overhaul of federal immigration law in decades.
Frank Sharry, head of the immigration advocacy group America's Voice, said Rubio's comments made him "anxious" and cautioned that the lawmaker must avoid taking his message too far.
"Rubio is uniquely qualified to talk to conservatives about immigration reform," Sharry said. "On the other hand, if he thinks now that he's the face of immigration reform, that he's going to drive this bill in a direction that makes it less palatable for the progressive coalition that created the political space for reform, it's going to be a huge problem."
Rubio, however, has said that failure to heed his concerns could endanger the prospect of overhauling immigration law, an issue that has historically divided conservatives and the American left.
As the most high-profile conservative in the bi-partisan "Gang of Eight" senators who labored for months to craft the legislation, Rubio will play a key role when it comes up for debate in the Democratic-controlled chamber.
Selling the bill is a delicate balancing act for Rubio. His comments are intended to build trust with conservatives, who have traditionally opposed immigration reform, but he must avoid alienating Democrats, whose support is also vital for passage.
While enactment of the bill could help Rubio's overall presidential prospects, it could cost him support in crucial early-voting conservative states such as Iowa and South Carolina.
Rubio has won credit from Democrats for backing a path to citizenship for undocumented immigrants. Pushing the economic case for the bill's passage, Rubio argues that new visa programs for foreign workers could boost America's global competitiveness, especially in the technology sector.
Yet he has often qualified his support by making clear he favors toughening the enforcement provisions of the bill, which are seen as politically necessary to get many Republicans' support for giving legal status to undocumented immigrants.
In a town hall-style appearance on Fox News last week, Rubio said he wanted to see the bill go farther to boost security on the southern U.S. border with Mexico. Asked about conservatives' mistrust of the Obama administration on border enforcement, he replied, "You've hit on the exact point."
Rubio promised to consider revisions that would put Congress in charge of creating a border security plan rather than the Department of Homeland Security as currently called for in the bill. "Unless we get people's confidence that we are going to secure the border, this bill won't pass," he said.
SELLING TO CONSERVATIVES
As a frequent guest on conservative radio and television programs, Rubio has a broad platform. And his arguments have converted some talk show hosts, including Mike Gallagher who acknowledged recently that he had been won over to supporting the central focus of the bill.
But David Leopold, general counsel for the American Immigration Lawyers Association, which supports immigration reform, said Rubio was "pushing the envelope."
"Rubio needs to back up this bill," Leopold said. "To offer a bill, to sit down and roll up your sleeves and write a bill with Democrats - I'm not going to discount that that took guts - but in that same vein, he needs to go into his caucus and sell this bill as a champion and not as a critic."
Marshall Fitz, director of immigration policy at the liberal Center for American Progress, which supports immigration reform, said some of Rubio's comments were "off-putting."
But he added that Rubio's approach might turn out to be a shrewd way of reaching out and showing fellow conservatives that he is listening to their concerns. "He does have a difficult role in trying to sell this package to conservatives."
Rubio's spokesman Alex Conant emphasized that the lawmaker was not the only member of the Gang of Eight to say publicly that more work was needed to pass the bill in the Senate.
Democratic Senator Robert Menendez, a Cuban-American from New Jersey, told Univision in an interview that the supporters "don't currently have" the minimum 60 votes needed to pass the bill in the Senate.
"After the last four or five months, I don't think anybody should question Senator Rubio's commitment to passing immigration reform," Conant said. That commitment is "why he says we need to be open-minded about criticisms and we need to be open-minded about improving the bill," Conant said.
Fitz warned that Rubio "may be creating expectations that aren't going to be able to be met" in terms of promises to toughen border security.
Other Republican members of the Senate's Gang of Eight, including Senators John McCain, Jeff Flake and Lindsey Graham, also are expected to push for the bill within their party.
"I think Rubio is critical," said Rebecca Tallent, a former aide to McCain who is now director of immigration policy at the Bipartisan Policy Center think tank, which supports immigration reform.
"He speaks to a lot of the folks who wouldn't necessarily be inclined to vote for something like this but Senator McCain brings the historical context and has a leadership role within the party and is good at building coalitions,"
In at least one recent closed-door meeting with Republicans, Rubio was even more blunt than he has been in public about what he sees as the bill's flaws, according to participants.
"The message was not, 'Listen, I've come up with a really good bill that we need to support it.' It was, 'This is a terribly flawed bill but it moves the ball forward and over the next weeks and months, it will move ever rightward,'" said a conservative who attended the meeting and spoke on condition of anonymity.