As a flamboyant veteran of Washington and New York City politics, the campaign strategist Roger J. Stone Jr. has been in any number of knock-’em-down scrapes over the years, reaching back four decades to his early days as a self-described “dirty trickster” in the Nixon administration.

But now Mr. Stone, a veteran adviser to President Trump who has long cut a piratical figure on the political scene, appears to be engaged in his stiffest fight yet: the one for his own legal future.

On Friday, a stream of developments in the special counsel investigation underscored his peril. An old friend — a former procuress from New York whom Mr. Stone has employed as an administrative worker — testified about him to the federal grand jury hearing evidence in the inquiry. Another old friend, a New York City radio host, has been subpoenaed to appear before the same grand jury. And one of his close aides was held in contempt of court for ignoring his own subpoena, though the order was stayed.

For months now, Mr. Stone, 65, has been a key focus of the inquiry by the special counsel, Robert S. Mueller III, into whether any Trump associates worked with Russian operatives who were secretly trying to tip the election in Mr. Trump’s favor. Mr. Stone is central to that question because he appeared to have advance knowledge of some of the moves that Russian hackers were making.