WASHINGTON – Buoyed by President Bush, supporters of immigration legislation established command in the Senate on Tuesday, brushing aside potentially crippling challenges to a bill that blends tougher border enforcement with a path to citizenship for millions in the United States illegally.

“It was a good way to start,” said Sen. Ken Salazar, D-Colo., as a shifting bipartisan coalition held firm against attacks from the left and the right.

On a vote of 55-40 that crossed party lines, the Senate rejected an appeal from Sen. Johnny Isakson, R-Ga., to require the border be secured before other immigration law changes could take place.

Anything less would mean “a wink and a nod one more time to those who would come here” unlawfully, said the Georgia Republican. The bill’s supporters said he had it backward. “We have to have a comprehensive approach if we’re going to gain control of the borders,” said Sen. Edward M. Kennedy, D-Mass., echoing Bush’s remarks of the night before.

Sens. Olympia Snowe and Susan Collins, R-Maine, voted against the Isakson proposal.

Hours later, Sen. Byron Dorgan, D-N.D., made an unsuccessful effort to exclude foreigners and recent illegal immigrants from a new guest worker program that could provide jobs for millions over the next decade. “This bill is going to allow illegal workers to come in stamped as legal,” he said, but the vote was 69-28 to scuttle his amendment.

Compromise averted a third showdown, when the bill’s critics and supporters agreed to deny illegal immigrants any chance at citizenship if they had been convicted of three misdemeanors or a felony.

The maneuvering took place at the beginning of what Senate leaders predicted would be a lengthy debate over the most significant changes in immigration law in two decades, an election-year issue that has laid bare deep divisions inside both parties and sparked street demonstrations across the country.

The Senate bill provides additional funds for border security, the guest worker program, an eventual opportunity at citizenship for most of the 12 million illegal immigrants in the country and a tougher program of enforcement to prevent the hiring of illegal workers.

The Senate accepted two changes during the day, one by Sen. Jeff Bingaman, D-N.M., to limit the guest worker program to 200,000 individuals a year, the other by Sen. John Kerry, D-Mass., to authorize the hiring of 1,000 additional Border Patrol agents as well as the purchase of new helicopters and boats.

Senate passage appears likely by Memorial Day.

Republicans and Democrats alike heralded Bush’s Monday night Oval Office speech as a turning point, at least as far as the Senate was concerned. The president announced plans to deploy as many as 6,000 National Guard troops in states along the Mexican border.