As Iranian negotiators spoke in positive tones about their resumed nuclear negotiations with the big powers, Congressional lawmakers in Washington introduced legislation on Wednesday that would greatly expand the sanctions on Iran, amounting to what both supporters and critics said would be like a commercial trade embargo if fully carried out.

The bipartisan measure, which was expected to pass both the House and the Senate, would build on existing laws that restrict business dealings with Iran, widen the list of blacklisted Iranian companies and individuals and potentially block Iran’s access to its foreign bank assets held in euros. That access has been one of the country’s few remaining avenues for repatriating profits that are not held in dollars, which have been greatly constricted by other sanctions.

The legislation, named the Nuclear Iran Prevention Act, also would penalize foreign companies and individuals that violate the American sanctions by threatening them with restrictions on doing business with the United States, a coercive tactic integrated into earlier, narrower sanctions.

Representative Ed Royce, a California Republican and chairman of the House Foreign Affairs Committee, and Representative Eliot L. Engel of New York, the committee’s ranking Democrat, who jointly introduced the legislation, said it was intended to prevent Iran from acquiring nuclear weapons ability and to penalize the country for what they called its “gross human rights violations.”