Democrats have been pressuring the Trump administration to be more hawkish on China and not to settle for a quick deal to end the trade dispute. Senator Ron Wyden of Oregon, the top Democrat on the committee, tried unsuccessfully to get Mr. Lighthizer to commit to keeping tariffs in place until China had demonstrated that it could keep its promises. Mr. Lighthizer refused to reveal whether the United States was pushing for such a policy, or whether it would lift its tariffs altogether once the deal was struck.

The United States wants China to commit to lowering trade barriers, ending the forced transfer of intellectual property in deals with American companies and scaling back subsidies of state-owned enterprises. It also wants China to promise to end competitive devaluation of its currency and be more transparent on foreign exchange matters.

While Mr. Trump and other top economic advisers like Steven Mnuchin, the Treasury secretary, and Larry Kudlow, the director of the National Economic Council, have expressed optimism that a deal is close, Mr. Lighthizer has been far more cautious in his assessment of the negotiations.

Last month, Mr. Mnuchin said in the Oval Office that an agreement was reached on currency, but Mr. Lighthizer said that while there had been progress on that front, nothing had been finalized.

“Nothing is ever done until everything is done,” he said.

“Lighthizer offers a far more grounded prognosis of the likelihood of success than President Trump and others in the administration recently have,” said Henrietta Treyz, director of economic policy at the investment advisory firm Veda Partners. “Without fail, Lighthizer makes sure to acknowledge that failure and success are both options with China.”

Mr. Lighthizer told lawmakers on Tuesday that China was pushing hard for all American tariffs to be removed, but that no decision had been made yet. The United States wants to maintain the right to impose tariffs if China is not living up to its commitments, raising the prospect that Mr. Trump could again levy taxes on Chinese goods if he grows unhappy with China’s behavior.

Figuring out how to enforce the trade agreement continues to be one of the biggest obstacles. Mr. Lighthizer on Tuesday outlined a situation in which American and Chinese officials would meet throughout the year and raise concerns of companies in their respective countries. The Trump administration had been pushing an approach that would force China to abdicate its ability to retaliate against new American tariffs. However, Mr. Lighthizer suggested that either country could impose tariffs if obligations created by the agreement were not being met.