CARLSBAD, Calif. -- Republican presidential candidate John McCain on Tuesday urged Congress to adopt a sweeping overhaul of the nation's immigration laws and challenged critics to offer a better alternative.

More Coverage: Presidential Campaign | Immigration Debate

The Arizona senator criticized his leading rivals, former Massachusetts Gov. Mitt Romney and former New York Mayor Rudy Giuliani, when a reporter asked about their opposition to a Senate compromise brokered earlier this month.

"Anyone who says we shouldn't pass this legislation, I would ask for their proposal," McCain said. "I have not heard their recommendation."

McCain said the U.S. cannot afford to continue allowing an estimated 12 million people to live in the country illegally. The Senate plan would establish a path to citizenship.

McCain repeated his plea for immigration reform minutes later before hundreds of technology executives at a conference sponsored by The Wall Street Journal.

Bush Backs Plan

President George W. Bush has continued his drumbeat for passage of the plan. He urged lawmakers in Congress Tuesday to "put politics aside and put courage first" by signing on to the new bill.

It's time for Congress to "do what's right, not what's comfortable," Bush said. He made his comments at the Federal Law Enforcement Training Center in Georgia.

"The fundamental question is: Will elected officials have the courage necessary to put a comprehensive immigration plan in place?" Bush asked against a backdrop of a huge American flag.

He described his proposal -- which has been agreed to by a bipartisan group of senators -- as one that "makes it more likely we can enforce our border and at the same time uphold the great immigrant tradition of the United States of America."