House Budget Committee Chairman Paul Ryan (R-Wis.) is planning to endorse in GOP primaries for Congress — and could side against incumbent Republicans, according to a report.

In a memo to supporters of Ryan's Prosperity Action PAC, Ryan lays out a mission of supporting candidates who will support his conservative agenda over those who won't.

"We need to replace timid Members of Congress with bold conservatives who know what needs to be done," he says in the memo, which was obtained by The Daily Caller.

The move comes as a surprise because Republican leaders, eager to hold on to their majority in the House, usually support sitting members who belong to their party. Even when more conservative candidates challenge an incumbent Republican in the primary, the incumbent advantage makes the sitting member a safer bet.

Efforts to support conservative challengers over sitting members usually fall to outside groups such as the Club for Growth.

Ryan has built a brand in the House — and his PAC — around his hardline budget proposal, which would drastically reduce the size of government by reining in entitlements. That budget passed the House in April but stalled in the Senate.