Rep. Chris Van Hollen has agreed to play the part of GOP vice presidential candidate Paul RyanPaul Davis RyanPelosi: 'Thug' Putin not welcome in Congress Interior fast tracks study of drilling's Arctic impact: report Dems unveil slate of measures to ratchet up pressure on Russia 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 KerryKerry: Trump attacks on Biden are 'unbecoming of a president' Congress has five ways to show American power against Russia Sunday shows preview: Questions linger over Trump-Putin summit MORE (D-Mass.) will be playing the part of Romney in President Obama's debate practice.