The ranks of forgotten Republicans are growing. Some were forced out, such as Tim Pawlenty, a former two-term Minnesota governor who lost this week's bid for a political comeback. Some, such as the retiring Republican Sen. Bob Corker, chose to leave on their own. Others still serve, but with a muted voice. Whether members of Congress, governors or state party leaders, they are struggling to fit into President Donald Trump's Republican Party. The expanding list of marginalized GOP leaders underscores how thoroughly Mr. Trump has dominated — and changed — the Republican Party in the nearly two years since he...