With the Chinese now making only vague concessions on structural reforms, Mr. Lighthizer would prefer to stay tough and let the bite of American tariffs persuade them to make bigger promises. While he has shared with Mr. Trump the view that the Chinese are not ready to make significant concessions, the president has continued to press for a deal.

Jeff Emerson, a spokesman for the United States trade representative, denied that Mr. Lighthizer had ever been frustrated with the president, saying that he and Mr. Trump were in complete agreement on the negotiating strategy with China.

“President Trump has never insisted that Ambassador Lighthizer make a deal with China and has instead told Ambassador Lighthizer he is interested only in making a ‘great deal’ with China that addresses structural issues,” Mr. Emerson added. “Anyone claiming otherwise either does not know what they are talking about or is deliberately pushing falsehoods.”

Mr. Lighthizer is well respected in Washington, with decades of experience in private legal practice and government, including leading trade talks with Japan in the 1980s. In trying to pin down the Chinese on making ambitious reforms to their economy, he has an unenviable task.

Many business groups and China watchers credit the Trump administration with forcing China to finally come to the table and address longstanding concerns, including its treatment of American companies. And they say the administration is poised to secure greater concessions than previous administrations have.

“Bob Lighthizer has made it a priority to get this China deal right,” said Myron Brilliant, an executive vice president and the head of international affairs at the U.S. Chamber of Commerce. “He understands that this is the moment in time in which there’s an opportunity to make a real change in the trajectory of the U.S.-China relationship, and he’s focused on the bottom line: what he can get from the Chinese that will improve the U.S.-China economic relationship.”

Mr. Lighthizer, who will testify before the House Ways and Means Committee, is expected to face tough questions from lawmakers about the state of the negotiations and whether he feels comfortable staking his legacy on the evolving China deal. While many Republicans are eager for the trade war to end, they are warning the administration not to settle for a weak deal. Democrats are already trying to outflank Mr. Trump on China and are gearing up to call any agreement that does not include structural changes a failure.