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“Talk about one step forward and two steps back,” Mr. Sharry said.

Immigration has proved to be an intractable issue for lawmakers in recent years. Efforts to pass legalization bills failed in 2006 and 2007, despite backing from Mr. Bush. Some border security measures also have passed, though they were later watered down.

This year, Democratic leaders in the Senate have outlined a plan that would include a multistep path to citizenship for noncriminal illegal immigrants now in the country, and rewrite the rules for future immigrants. The plan also includes more funding for certain security functions, and would bar illegal immigrants from gaining legal status until those benchmarks are met.

But Republicans said border security not legal reform must come first.

“How do you get 60 votes for a pathway to citizenship when the whole country is focused on broken borders?” said Sen. Lindsey Graham, South Carolina Republican, who had been working on a broad immigration bill with Senate Democrats but broke off those talks last month.

His colleagues have introduced several border security amendments to the emergency war-spending bill now pending on the Senate floor, including the McCain-Kyl proposal to add 6,000 troops, with 3,000 of them going to Arizona.

“You don’t need comprehensive reform to secure the border, but you do need to secure the border to get comprehensive immigration reform,” Mr. Kyl said on the Senate floor.

In their letter to Congress, Mr. Obama’s advisers said there is no precedent for Congress directing the president to deploy National Guard troops like that, and said such an extensive deployment could hurt the Guard as it participates in the wars in Iraq and Afghanistan.

Mr. Obama, meanwhile, will ask lawmakers to tack on $500 million to pay for his troop deployment and to boost federal customs, drug and immigration authorities.

The decision to send National Guard troops to the border was announced less than a month after Arizona enacted a law requiring police to check the legal status of those they encounter during their duties who they have reasonable suspicion to be in the country illegally. Racial profiling is specifically prohibited.

On Friday, a top federal immigration official warned that the federal government may not actually collect and deport those caught under the new law.

In the closed-door meeting with Republicans on Tuesday, Mr. Obama said he had read the Arizona law and still thought it has the potential to lead to civil rights violations.

But in Phoenix, Arizona Republican Gov. Jan Brewer credited her signing of the controversial new law for compelling Mr. Obama to act. Signing the law, Mrs. Brewer said in a statement, “clearly ignited the talk of action in Washington for the people of Arizona and other border states.”