Tulsi Gabbard, a former House member who ran for president in 2020 as a Democrat and then left the party, on Wednesday embraced the idea of joining Donald J. Trump as his running mate on the Republican ticket. “If asked to serve in that way, I would be honored to do so,” Ms. Gabbard said in response to a question from the audience during a speech at the Richard M. Nixon Library and Museum in California. She was promoting her book about her estrangement from the political left...