BOSTON--Ralph Nader continued his uphill struggle as a third-party presidential candidate yesterday, appealing to a near-capacity FleetCenter crowd to help him build "a progressive political majority in this country."

The rally, which had been heavily promoted for weeks on local college campuses, attracted about 12,000 supporters and a stage full of liberal celebrities.

Organizers hoped the FleetCenter bash would solidify Nader's support among young progressives in the area, as well as attract media coverage.

Nader's principal theme was his exclusion from tomorrow's presidential debate at U-Mass Boston.

"Never again should we allow this to happen in future elections," Nader said. "Never again."

During tomorrow's debate, which begins at 9 p.m., Nader said he will join protestors outside the converted gymnasium that will serve as a studio.

The Green Party nominee will spend several pre-debate hours at Harvard, addressing the ARCO Forum and then staying to participate in a panel on the living wage, his advisers confirmed.

The presidential debate's sponsor, the bipartisan Commission on Presidential Debates (CPD), requires that candidates have the support of at least 15 percent of those surveyed in a number of national polls.