It's understandable that so many Republicans are all but begging Paul Ryan to step up as a consensus choice for speaker of the House. It's even more understandable that Ryan wants to demur. And despite the mess his GOP colleagues find themselves in, the Wisconsin congressman should follow his first instinct. Because if Ryan enters the speaker's race, he'll either fail as populist conservatives trash his less-than-perfect voting record, or he'll succeed and spend the next several years herding cats instead of advocating big and important entitlement reforms.

For all Republicans' talk of balanced budgets, the national debt, and reducing government spending, Republicans don't have a very good record when it comes to actually reforming two huge spending programs that are big drivers of America's long-term debt: Social Security and Medicare. Ronald Reagan basically caved to the Democrats on both programs. When George W. Bush wanted to reform Social Security, he found that the enthusiasm among the Republicans controlling both houses of Congress just wasn't there.

That's what Ryan found when he began introducing his budget blueprints containing ambitious and controversial Medicare reforms. These initial forays into addressing entitlements attracted little support. But over time, as Ryan climbed the ladder to run first the House Budget Committee and then Ways and Means, versions of those wild-eyed ideas became official Republican budgets that commanded majority support in the GOP House.

It's impressive how much Ryan has accomplished. His aggressive budget plans have taken such a hold as the new Republican consensus in favor of entitlement reform that the party's 2012 presidential nominee, Mitt Romney, endorsed the broad strokes of Ryan's plan and ultimately the man himself when he chose the Wisconsin congressman to be his running mate. Jon Huntsman, the "liberal" in the Republican primaries, was a supporter of the Ryan plan.

Still, the pro-Ryan consensus was always shaky. Democrats accused Ryan of wanting to privatize or even abolish Medicare. Republicans were nervous when they lost a 2011 special election to a New York Democrat who ran aggressively against Ryan's reforms. They muttered when Democrats ran ads showing Ryan pushing a wheelchair-bound old woman off a cliff. Many of their constituents benefited from Social Security and Medicare, as older voters are a key GOP constituency.

Now that consensus is under attack. Two major Republican presidential candidates, Mike Huckabee and Donald Trump, have openly questioned the need for Ryan-style reforms. Huckabee has said we should cut congressional pensions before entitlement spending. Trump has said he has the financial acumen to fix these programs without changing their benefits. Both have questioned whether Social Security and Medicare are really entitlements at all.

Trump has for months been the frontrunner in the race for the Republican nomination, leading in polls nationally and in the early states. Chris Christie, the GOP presidential candidate running on Ryan-like reforms and inconvenient truth-telling about the sad financial state of our entitlement programs, has so far gone nowhere.

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.

More recently, Ryan's 2013 deal with Democratic Sen. Patty Murray was decried as a capitulation on spending concessions Republicans had already won. The conservatives who brought down Boehner and stopped Kevin McCarthy want to keep members from losing their committee assignments when they vote against leadership. Justin Amash and Tim Huelskamp, two important members of this faction, were kicked off the House Budget Committee for voting against the Ryan budget. Some blame Ryan himself.

Ryan has counterarguments to all these points. But wouldn't his talents be better applied to defending conservative policy ideas rather than his own record? Unlike the conservative rebels in the House now, Ryan climbed up the ranks in the traditional ways and has never been in a thorn in leadership's side. This has paid dividends for conservatives at times, but is not the best fit for the current mood.

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.

On some level, Ryan likely senses all this. He's always been reluctant to seek promotions and seems to fancy himself a Jack Kemp-like policy entrepreneur just waiting for a Reagan to become president to sign those policies into law.

I've made similar arguments when Ryan's name has been floated for higher office before, including the presidency and the vice presidency. Ryan made it out of the 2012 campaign relatively unscathed. Why take that risk a second time?