Opponents of campaign finance legislation are digging in their heels in the Senate, determined to kill a measure that finally has the support of a narrow majority of lawmakers.

"This bill is not going to pass. Not today, not tomorrow," Sen. Mitch McConnell, R-Ky., said on Tuesday, shortly after the legislation had cleared its first hurdle. The measure survived on a test vote of 51-48, a narrow majority but well short of the 60 votes that will be needed to overcome a filibuster led by its Republican foes.The next test is likely to come late Wednesday or perhaps Thursday, on a proposal crafted by Maine Republican Sen. Olympia Snowe designed to curtail union spending and attack ads in political campaigns.

"We should be putting our heads together, not building walls between us with intractable rhetoric and all-or-nothing provisions," Snowe said Tuesday evening as she appealed for support from GOP critics of her proposal.

The campaign finance legislation, developed by Sens. John McCain, R-Ariz., and Russell Feingold, D-Wis., would ban unregulated "soft money" that flows to national political parties from corporations, labor unions and individuals. It also would impose fresh curbs on advertisements that attack candidates but escape regulation because they are presented as "issue ads" not covered by existing election law.

Within moments of the roll call on Tuesday, Senate Majority Leader Trent Lott exercised his prerogative to slow the bill's passage.

"Here we go again," lamented Senate Minority Leader Tom Daschle, D-S.D.

President Clinton - whose 1996 campaign fund raising has spawned congressional investigations and criminal indictments - issued a statement saying that "only the obstruction of a minority stands in the way of a law that would strengthen our democracy."