WASHINGTON—The White House said Friday that President Donald Trump is weighing his legal options in advance of former FBI Director James Comey’s scheduled testimony before Congress next week, but experts said the president has little recourse to block the testimony.

In a widely anticipated event, Mr. Comey is scheduled to appear Thursday before the Senate Intelligence Committee, which is probing Russia’s alleged interference in the 2016 election—his first public appearance since Mr. Trump abruptly dismissed him on May 9. Before...