Republicans are in a populist mood and today’s populist conservatism cares a lot about curbing immigration and very little about entitlements. Ryan is on the opposite side of both of these issues. Exactly who counts as a “RINO” and who is a conservative true believer is in a state of flux.

For Ryan to enter the race for speaker with the tacit support of John Boehner and the establishment in this climate threatens to shatter the pro-reform consensus on entitlements. Ryan’s conservative credentials will be attacked as never before. He will be branded a RINO and an amnesty supporter. His big and important ideas on spending reform could well go down with him.

The conservative case against Ryan won’t be limited to immigration, however. He voted for most of the big spending and government growth of the Bush years, including the Medicare prescription-drug benefit and the TARP bailout. While conservative think tanks and magazines admire Ryan, these votes didn’t endear him to the Tea Party groups who are in some cases more directly connected to the grassroots…

As speaker, Ryan would move from primarily advocating entitlement reform to corralling Republican votes for debt-ceiling increases and other things that are unpopular with conservatives. That threatens to make his better ideas less popular with conservatives.