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The Senate's Republican leader says he is unsure whether he will vote for the immigration bill President Bush strongly supports, underscoring the measure's precarious status.

Sen. Mitch McConnell, R-Ky., has long called for an immigration overhaul, saying the current situation is deeply flawed. And as the Senate minority leader, McConnell is central to shepherding legislation the president wants.

But in an interview with The Associated Press on Thursday, McConnell said he would not decide how to vote on the measure until a long series of amendments are disposed of next week.

"The bill on the merits is a mixed bag," said McConnell, who had brushed aside reporters' questions on immigration Tuesday and Wednesday. "I'm not uniformly enthusiastic about it."

"At the end of the process," he said, "we're going to have to make a call as to whether this is an improvement over the status quo. I'm not ready to make that call yet."

McConnell said it is unclear whether the bill's supporters can muster the 60 votes eventually needed to allow a final roll call on the bill in the 100-member Senate. His chief goal, he said, has been to see that all Republicans are treated fairly and allowed to be heard.

"In the end, I frankly don't know whether this thing will fly or not," McConnell said. "But we will have given it our best shot."

McConnell's ambivalence has been known to colleagues, but Thursday's comments about his misgivings were especially blunt and specific. To a large degree, they reflect the deep divisions the issue has carved in the Republican base.

Many business groups, hungry to fill low-wage jobs, support the bill. Many social conservatives, backed by talk show hosts, denounce it as amnesty for illegal immigrants.

The Senate bill would tighten borders and workplace enforcement, create a new guest worker program and provide pathways to legal status for most of the estimated 12 million illegal immigrants in the country. The House has yet to draft legislation.

The immigration debate has squeezed many politicians, but perhaps none more so than McConnell, a strong White House ally. Some see the legislation as Bush's last hope for a major domestic achievement, and McConnell himself has repeatedly said an immigration revision is one of the "big things" a divided government can achieve.

But most Senate Republicans thus far have refused to embrace the bill. And some party strategists think voters in 2008 will reward those who oppose giving illegal immigrants lawful status.

McConnell is "not riding two horses, he's trying to decide which horse to ride," Sen. Arlen Specter, R-Pa., a bill supporter, said in an interview. "He has a very difficult role, with the caucus so badly split. He has a duty to represent the caucus."

McConnell has proceeded cautiously, demanding that Republican senators be allowed to offer about two dozen amendments when Democrats tried to cut off debate sooner. He has declined to pressure colleagues, letting other party members step out front to defend and push the measure.

Some Republican backers of the bill grumble that McConnell has been too tepid. But others defend his approach, saying browbeating colleagues on such an emotional topic might backfire.

"I think he and Trent are dealing with it about as well as leadership could," said Sen. Lamar Alexander, R-Tenn., referring to McConnell and Senate Republican Whip Trent Lott of Mississippi.

Lott has been more outspoken in saying the bill is flawed but needed, and in criticizing those who denounce it. Conservative talk show hosts have blistered Lott in return, while leaving McConnell largely alone.

Still, the corrosive debate seems to have taken a toll on McConnell. On Tuesday and Wednesday, he refused to take reporters' questions on immigration, a rare move in the Capitol hallways for a man whose deadpan demeanor and calm, monotone voice seldom change.

Privately, some Bush allies say they wish McConnell would openly back the immigration bill. Publicly, colleagues who support the bill have placed McConnell's comments and actions in the best possible light.

"I take him at his word," said Sen. Lindsey Graham, R-S.C., noting that McConnell repeatedly has said the Senate bill is preferable to the status quo. "He has been helpful in making sure it comes back" for more amendments and votes, Graham said, referring to last week's hiatus that nearly doomed the bill.

McConnell agreed that keeping the process moving, without embracing or rejecting the bill, has been his aim. Pressuring reluctant Republicans to back the president "would be exactly the wrong way to operate on a bill of this type," he said. "It would have been counterproductive."

McConnell noted that earlier this month he voted for an unsuccessful amendment that would have eliminated so-called Z visas for immigrants who lack legal status. Bill supporters called it a killer amendment, but critics call the Z visas the key to granting amnesty to illegal immigrants.

"Substantively, there are things to like and things to not like about this bill," McConnell said.