WASHINGTON—Hunter Biden is willing to testify publicly before Congress, his lawyer said in a letter Tuesday, responding to a subpoena demanding that he appear as part of the House Republicans’ impeachment inquiry into his father, President Biden.

The three-page letter from the younger Biden’s lawyer, Abbe Lowell, marked the latest escalation in the monthslong standoff between the president’s son and GOP lawmakers. The more aggressive approach seeks to ward off the prospect of a closed-door hearing after which Republican committee members could cherry-pick parts of Hunter Biden’s testimony, but it also augurs a more public phase of his travails.