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.

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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.