Manchin is staking out some ground. Recall that once the President is replaced the Vice President’s office becomes open and any replacement will require a majority vote in the Senate. The Senate is currently 49 Republicans, 47 Democrats, and 4 Independents. They’ll need all four indies for approval because the office being vacant, the VP is no longer a tie-breaking vote. And that gives Manchin the sort of leverage a politician dreams of.
The 25A requires a majority of the cabinet and 2/3 of the House and Senate - if the President resists.