Rep. Chris Van Hollen has agreed to play the part of GOP vice presidential candidate Paul RyanPaul RyanThe Hill Interview: Budget Chair Black sticks around for now Gun proposal picks up GOP support GOP lawmaker Tim Murphy to retire at end of term MORE in debate prep, according to an Obama campaign official.

Van Hollen's stand-in role is fitting — the Maryland Democrat serves as the ranking member on the House Budget Committee. Rep. Ryan (R-Wis.) is the chairman of that panel.

Debate preparations for Vice President Biden have yet to begin, the official noted. The debate between Biden and Ryan is scheduled for Oct. 11.

Sen. John KerryJohn Forbes KerryFor the sake of national security, Trump must honor the Iran deal Bernie Sanders’s 1960s worldview makes bad foreign policy DiCaprio: History will ‘vilify’ Trump for not fighting climate change MORE (D-Mass.) will be playing the part of Romney in President Obama's debate practice.