Washington ? This week’s bipartisan agreement on an immigration reform bill may have been a substantial achievement, but lawmakers and advocates from the left and right attacked it Friday and vowed to change or scuttle it.

If the compromise plan is to become law, the senators who drafted it – along with President Bush – will have to address complaints from a spectrum of critics, from immigrant advocates hoping to relax provisions of the guest worker program to conservatives who hope to defeat the bill because they see it as an amnesty for the 12 million illegal immigrants in the United States. But support for the measure could dwindle if key provisions are substantially altered.

The campaign to sell the comprehensive reform legislation officially begins Monday afternoon when the bipartisan group, led by Sens. Edward Kennedy, D-Mass., Jon Kyl, R-Ariz., and others offer their 380-page measure. They are expecting a favorable reception and eventual passage in the Senate.

Sen. Lindsey Graham, R-S.C., called the proposal the “last, best chance we’ll have as a Congress” to enact immigration reform.

But advocates for immigrants have shown more measured optimism, pledging to seek key changes but lauding the effort overall as an important first step. But some warned against assuming that an agreement among senators as diverse as Kennedy and Kyl was a harbinger of easy passage in the Senate.

“To say there will be overwhelming support is unrealistic and a bit idealistic,” said Marshall Fitz, advocacy director for the American Immigration Lawyers Association. “It’s not at all certain that as a final package this could garner the 60 votes” needed to block a filibuster. “I think that’s an open question.”

The bill would establish a merit-based, temporary guest worker visa program for new immigrants, while eliminating the backlog of legal immigrant applications and legalizing the status of undocumented workers currently in the country.

These and other components of the bipartisan bill are controversial enough that those who drafted it will have to work to secure the support of their fellow party members.

If the measure does get past the Senate, even bigger challenges likely await in the House, where critics of the Senate bill have been more vocal, and political camps are more sharply divided.