* Judiciary Committee opens amendment marathon
* Republicans express skepticism and promise long process
* House negotiators still face unresolved issues
By Richard Cowan and Thomas Ferraro
WASHINGTON, May 9 (Reuters) - A U.S. Senate committee on Thursday launched a weeks-long effort to pass a comprehensive immigration bill with a warning from the panel's top Republican that he would make the process as long and "arduous" as possible.
"I plan to ask many questions throughout this process," Iowa Senator Charles Grassley warned the Democratic-controlled Senate Judiciary Committee. "I want to know how the bill doesn't repeat the mistakes of the past." Grassley, in a statement, promised an "arduous" and "robust" debate.
An outspoken critic of comprehensive immigration legislation, Grassley has introduced nearly 80 amendments, many of them inevitably objectionable to the sponsors of the measure and to President Barack Obama, for whom immigration law reform is a top priority.
The opening day reflected both the deep divisions and high hopes surrounding a measure that would put 11 million illegal residents on a path to citizenship and totally revamp the criteria for who gets into the United States and for what purpose.
Before the session began, a group of spectators with the words "Campaign for Citizenship" emblazoned across their white T-shirts stood in a circle in the hearing room, their hands raised above their heads, for a silent prayer.
The Rev. Alvin Herring of Washington, D.C., told Reuters: "It's going to take prayer and it's going to take us acting on our prayers" in order to get immigration legislation enacted.
The panel's 10 Democrats and eight Republicans were prepared to argue over as many as 300 amendments to a bill that was crafted by the bipartisan "Gang of 8" senators.
Some of the proposed amendments are designed improve the measure, while others are seen as ways to possibly kill it. Four of the senators who crafted the complex measure are on the committee, and these two Democrats and two Republicans have agreed to jointly oppose any amendment seen as a "poison pill."
The panel's work could stretch through May, and if it succeeds in approving legislation, the full Senate is likely to debate it throughout June.
But several Republicans on the committee - and in the full Senate - are skeptical of legalizing 11 million people who either came into the United States illegally over the past 27 years or overstayed their visas.
Instead, they want a more limited immigration bill that mostly concentrates on other aspects of the legislation, including further securing U.S. borders and creating more visas for skilled workers to help American high-tech companies.
Meanwhile, negotiations on a bill in the more conservative Republican-led House of Representatives slogged on.
According to one House source familiar with the negotiations, disagreements remained over several important policy matters, including how many low-skilled workers should be allowed into the United States to perform jobs ranging from cooks and hotel maids to construction workers.
This was one of the most contentious issues during negotiations between the AFL-CIO labor organization and the U.S. Chamber of Commerce in the run-up to introduction of the Senate bill.
The source said it was still unclear when the House members would unveil legislation.
Marshall Fitz, an immigration specialist at the Democratic-leaning Center for American Progress, said that besides policy disagreements, House members also face many political and strategic questions.
There is widespread belief that for a bill to pass the House, it will have to be significantly more conservative than the Senate bill, which some immigration advocacy groups already complain contains overly rigorous requirements.
"Democrats in the (House) group are rightly wary of signing onto a bill that is significantly to the right of what the Senate is doing," Fitz said.
Democratic Senator Charles Schumer of New York, who helped write the Senate bill, defended it as a "fair bill that no one gets everything they want, but at the end of the day it will mean a dramatic improvement for the American economy, the American people" while also updating U.S. immigration policy in a way that discourages illegal immigration.
But Schumer also said, "There are many who want to kill this bill," as he pleaded with his fellow committee members to instead try to improve it.
Editing by Fred Barbash and Jackie Frank