The slippage is prompting a White House review of its strategy, administration and congressional officials said.
House Democratic Whip David Bonior of Michigan, the second-ranking Democrat in the House and an opponent of the legislation, said he counts 128 of the House's 211 Democrats as now opposed to the measure and "a huge number of people who are undecided."
The legislation would grant China permanent normal trading relations as a step towards its entry into the World Trade Organization. Now, China's status is renewed annually.
Bonior said that at least 30 Democrats who voted last year to renew trade benefits for China now say they will vote against making the preferences permanent.
Although the measure still has strong Republican support, the opposition by organized labor has seriously cut into potential Democratic election-year backing, particularly in the House, where all seats are up for election every two years.
China's recent ratcheting up of its threats toward Taiwan and a State Department report last week citing continued human rights abuses have also cut into support.
House Republican leaders have said they cannot promise a victory unless Clinton can rally at least a majority of House Democrats. Bonior predicted as many as two-thirds of House Democrats will oppose the measure.
As a result, the White House and congressional supporters of the trade deal, who had planned to make their stand first in the House, are now considering trying to move it first in the Senate instead. Trade legislation traditionally has had more bipartisan support in the Senate, where only a third of seats are up every two years.
A Senate win would give the trade measure some momentum going into a more difficult House vote, the theory goes. The administration, which has predicted that it would prevail in both chambers, is now not sure it can win a House vote, said administration officials and congressional Democratic proponents of the bill who spoke on the condition of anonymity.
The administration and business groups have waged a spirited and intensifying campaign to build support for the measure, claiming it would give U.S. businesses and farmers new access to Chinese markets.
But labor groups and human rights activists protest worker conditions in China and labor is also fearful that expanded trade would threaten U.S. manufacturing jobs.
A new poll released Thursday by the Pew Research Center for People and the Press suggested Americans are generally supportive of free trade with other nations, but oppose granting normal trade relations with China by a 2-to-1 margin.
When people were asked what should be given the most consideratin when deciding U.S. policy about trading with other countries, three-fourths said protecting the jobs of American workers and keeping the U.S. economy growing. Only a fifth said helping the interests of U.S. businesses overseas should be a top consideration.
By White House count, President Clinton has already invited about 60 members to the White House to sell the pact, and more meetings are planned for the coming days. Commerce Secretary William Daley and Agriculture Secretary Dan Glickman also plan to lead undecided delegations of lawmakers to China this spring.
Such trips "can be effective," Bonior conceded, but he told a group of reporters that he feels momentum is moving his way.
Proponents are hoping for a vote before summer political conventions and argue that they are making some progress in lining up support.
For instance, they cite this week's decision by Rep. Greg Ganske, R-Iowa, a former foe of China trade benefits, to support the legislation, saying his change of heart "added momentum to the effort."
Supporters are hard pressed to name any other converts.