Washington ? The White House and a bipartisan coalition of senators on Tuesday defeated a vigorous assault on a proposed temporary guest-worker program as they battled attempts to dismantle a far-reaching immigration bill.

The guest-worker program, a centerpiece of the carefully crafted legislation, would allow U.S. businesses to bring in 400,000 foreign workers a year – and possibly as many 600,000 – to fill what businesses say is a chronic shortage of low-skilled workers.

As the Senate began its first full day of debate on the legislation, organized labor put its weight behind an amendment by Democratic Sens. Byron Dorgan of North Dakota and Barbara Boxer of California to kill the guest-worker program.

Senators defeated the measure 64-31, but the bill’s sponsors braced for further attacks on the program this week, including amendments to cap the number of guest workers at 200,000 a year, and possibly fewer. Pro-immigration groups and humanitarian organizations also want to put such workers on a path toward U.S. citizenship instead of forcing them to return home when their visas expire.

Boxer and Dorgan charged that admitting hundreds of thousands of guest workers would undermine wages, create a permanent underclass of foreign workers and possibly drive U.S. workers from their jobs. The bill’s backers argue that the measure contains adequate safeguards to protect U.S. employees and foreign guest workers.

“Why would anyone bring this kind of a program to the floor of the U.S. Senate?” Boxer declared. “This is a terrible idea, and it needs to be deleted from this bill.”

Senate Majority Leader Harry Reid, D-Nev., also indicated his support for curbing the program, calling the yearly influx of 400,000 workers “far too much.”

The Dorgan-Boxer proposal, identical to one that attracted 28 votes in the 100-member Senate just over a year ago, was a high priority for labor groups, which played a major role in helping Democrats seize control of Congress in the 2006 elections.

Twenty-eight Democrats voted for the amendment, along with two Republicans and one independent.

The battle over guest workers was only the first of many that face the bill’s sponsors as they embark on two weeks of debate on legislation that’s drawn attacks from left and right.

The measure also would legalize most of the estimated 12 million illegal immigrants, toughen border enforcement, curtail family-based immigration and establish a point-based merit system for accepting future immigrants.

Senators will continue debate all week before they begin a weeklong Memorial Day break. They’ll resume work on the bill for at least another week after they return on June 4.

President Bush has repeatedly insisted that a guest-worker program should be part of any immigration overhaul to create a legal pathway for needed foreign workers in jobs now filled by millions of illegal workers.

“Temporary workers are obviously needed,” said Senate Minority Leader Mitch McConnell, R-Ky. “We have almost full employment in this country. Most of the people who are coming into the country are coming here to work.”