U.S. trading partners have accepted the message of a new trade bill - that foreign markets must be open to American goods - and President Reagan should, too, a House committee chairman said Saturday.

Rep. Dan Rostenkowski, D-Ill., whose Ways and Means Committee has jurisdiction over trade, used the Democratic Party's weekly radio address to urge Reagan to sign the trade bill.The measure's fate, after three years of wrangling in Congress, is uncertain because Democrats have refused to remove a controversial labor-backed, plant-closing provision. The president's spokesman, Marlin Fitzwater, on Friday renewed a White House threat to veto the bill if the clause stays in.

The plant-closing clause would require businesses with at least 100 employees to give 60 days notice of factory shutdowns and layoffs. Business groups oppose it, contending the requirement infringes on the prerogatives of company managers.

Legislators are attempting to iron out differences between House and Senate versions of the 1,000-page trade legislation. The Democratic leadership seemed close to dropping the plant-closing provision last week, but stood firm after lobbying by organized labor and after the Commerce Department said the nation's trade deficit widened in February.

Rostenkowski said that even though Reagan refused to cooperate in writing the legislation, it addresses many of his concerns.

"Our legislative goal was a trade bill that was so strong our trading partners could not ignore it and so fair that the president would have to sign it. I think we've succeeded," he said.