New Delhi, May 22 -- The U.S. military said Wednesday that it boarded an Iranian-flagged oil tanker in the Gulf of Oman that was suspected of trying to violate the American blockade, the latest action by the Trump administration to try to push Tehran to reopen the Strait of Hormuz.

But President Donald Trump is facing his own pressure at home for shipping to resume through the vital corridor off Iran's coast. Fellow Republicans in Congress are battling political headwinds ahead of November's midterm elections as gasoline prices skyrocket and global energy markets churn.

Meanwhile, the Senate on Tuesday advanced legislation seeking to force Trump to withdraw from the Iran war, with a growing number of Republicans defying the president in...