climb on the bandwagon
To join a particular cause or political party: “When the party leader saw how popular the opposition was becoming, he decided to climb on the bandwagon and offer his full support to his opponent.”
The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.