Senator Adam Schiff of California, one of the Democratic Party’s leading antagonists of President Trump, has formed a new legal defense fund in recent days as Mr. Trump threatens him with federal investigations. As a member of the U.S. House, Mr. Schiff led congressional inquiries into Mr. Trump during his first term in the White House. He served as the lead impeachment manager in Mr. Trump’s first impeachment and then on the committee that investigated the Jan. 6, 2021, riot at the Capitol. Joseph R. Biden Jr., before he left office, issued pre-emptive pardons to the members of that committee...