You can break Paul Ryan's career -- and his policies -- into three separate periods.

There were the Bush years, in which Ryan cared about conservatism but not about deficits. These are the years when he authored a Social Security privatization plan that was so costly the Bush administration dismissed it as "irresponsible." These were the years in which Ryan voted for trillions of dollars in tax cuts and wars and Medicare drug benefits without demanding any way to pay for any of them. If a policy even arguably pointed in a conservative direction, he was an "aye" vote, no matter how extreme, or how fiscally irresponsible.

Then came the Obama years, in which Ryan remade himself as a conservative who cared only about deficits. The general thrust of his policies didn't much change, but now he promised to pay for everything, and condemned his party for its profligate past. It's during this period that Ryan began moving his suite of conservative reforms -- privatizing Social Security, block granting Medicaid, voucherizing Medicare, individualizing health care, investor-friendly tax reform -- into the broader framework of "Roadmap" budgets, which allowed them to be analyzed not so much as individual policies, but as contributors to deficit reduction.

This was a crucial insight for him. Deficit reduction occupies an unusual and special role in the Washington conversation. There are a lot of things you can do under the guise of deficit reduction that would be considered almost unspeakably extreme if you proposed them on their own. Devolving Medicare into capped vouchers feeding into private plans, for instance, would typically get you labeled a conservative radical. But if you're doing it to reduce the deficit, then it's making "hard choices."

It's during the Obama years that Ryan really became a leader in the Republican Party, but becoming a leader in the Republican Party came with certain compromises. Social Security privatization, for instance, was a bridge too far for the GOP, so though that appeared in Ryan's 2010 Roadmap, it was jettisoned in his 2011 budget. Similarly, offering specific health-care reforms began to look like a distraction in the fight against Obamacare, and so those were tossed overboard, too.

But he was able to keep his plan to move Medicare to vouchers, to block grant Medicaid, and to rewrite the tax code, and by loading his budget up with strict (but vague) caps that promised huge long-term budget savings, he was able to build a reputation as one of fairly few politicians willing to make the tough sacrifices required for deficit reduction.

And now we're in the third Ryan era, which is his vice presidential campaign. That's come with sacrifices so deep that they're more like contradictions. Where his budget keeps Obama's $716 billion in Medicare cuts, he's now attacking the very idea that you would cut Medicare for current seniors. Where his health-care reforms ended the tax break for employer-based insurance, he's now hammering Obamacare because the Congressional Budget Office says, in a worst-case scenario, that 20 million Americans might move from employer-based insurance to exchange-based insurance -- a small fraction of the transition Ryan was looking to trigger. Where he made his name being unusually specific about what and where to cut, he's now defending Romney for being unusually vague on those topics.

What's been somewhat unusual about Ryan is that he's managed to move from one era to the next without seeming to carry much baggage. Social Security privatization, for instance, is one of the most toxic ideas in American politics, but Ryan is rarely asked to answer for it, or even asked about it at all. Ryan's record on the deficit was, in the Bush years, as bad as anyone in politics, but that hasn't stopped him from building a reputation as a deficit hawk in the Obama years. Ryan's plans violated a number of Romney's policy promises, but that didn't stop him from becoming Romney's running mate.

This isn't necessarily a bad thing. Politicians should change their policies over time. The idea that legislators should be consistent over the decades, even as the world and our information about it changes, is an odd and probably damaging conceit. 

But Ryan's past isn't particularly distant. When he protests that his opponents are attacking him for a past iteration of his Medicare plan, he's protesting them for attacking a policy he offered last year, not in 1992. It's true that Ryan's current set of policies is more moderate than his 2011 proposals, and that his 2011 proposals were different than his 2005 proposals, but there's no evidence his underlying ideas have changed. It's more that who he'd had to compromise with, and how he needs to present himself, have changed.

An interesting thread in Thursday night's debate was that Biden didn't get trapped in the weeds of what Ryan supported when. Rather than argue about Ryan's latest budget, he wrapped all of Ryan's past and present ideas together and asked voters, who do you really think you can trust to preserve Medicare?

This short-circuited Ryan's typical rejoinder. Biden wasn't arguing that Ryan's 2004 Social Security privatization plan is in his 2012 budget, or that his 2011 Medicare plan is identical to Ryan-Wyden. He was simply saying that these were ideas Ryan had come up with quite recently, and voters should take them into account when considering whether to trust Ryan's instincts on these programs.

It was one of the debate's more devastating moments, in large part because Ryan didn't have a good answer for it. That's something he and his team need to think about. Whatever happens in this election, if Ryan wants to be president in 2016 or 2020, he's going to need to figure out how to explain his past ideas in a way that doesn't threaten his present ambitions.