WASHINGTON, Oct. 23 — President Bush is planning to issue a stern warning Wednesday that the United States will not accept a political transition in Cuba in which power changes from one Castro brother to another, rather than to the Cuban people.

As described by an official in a background briefing to reporters on Tuesday evening, Mr. Bush’s remarks will amount to the most detailed response — mainly an unbending one — to the political changes that began in Cuba more than a year ago, when Fidel Castro fell ill and handed power to his brother Raúl.

The speech, scheduled to be given at the State Department before invited Cuban dissidents, will introduce the relatives of four Cuban prisoners being held for political crimes. A senior administration official said the president wanted to “put a human face,” on Cuba’s “assault on freedom.”

In effect, the speech will be a call for Cubans to continue to resist, a particularly strong line coming from an American president. He is expected to say to the Cuban military and police, “There is a place for you in a new Cuba.”