When Donald J. Trump was first in the White House trying to strike a trade deal with China, Beijing tried to curry favor with the American president by banning all variants of the powerful synthetic opioid fentanyl.

Now, as China and the United States are deadlocked in a bitter trade war that has roiled the world, offering to do more to further stem the flow of fentanyl and the chemicals used to make it could once again be an option for China.

Beijing has for years used cooperation on fentanyl as leverage in its broader relationship with the United States. The move to ban all variants in 2019 came after President Trump accused China of not doing enough to stop the flow of the drug into the United States, and as the issue of fentanyl became increasingly tangled in a broader trade dispute.

China also agreed at that time to participate in joint law enforcement crackdowns on fentanyl with the United States. The moves led President Trump to praise the Chinese leader, Xi Jinping, for making “a wonderful humanitarian gesture.”

With the new Trump administration, China has faced even greater pressure to do more to crack down on the export of chemical precursors used to make fentanyl. The administration cited fentanyl as the reason for twice imposing additional tariffs of 10 percent on China in February and March. The Trump administration also closed a loophole on Friday that it said allowed small quantities of the chemical precursors to be shipped to the United States undetected.

Mr. Trump, who asserts that China has cheated on trade with the United States for decades, has also raised tariffs on most Chinese goods to a minimum of 145 percent. China has responded with similarly sky-high tariffs, vowing to “fight to the end” against what Beijing calls blackmail.