LET me be clear, as he likes to say: I believe that President Obama was entirely sincere when he ran for president as a fierce critic of the imperial executive. I believe that he was in earnest when he told supporters in 2008 that America’s “biggest problems” involved “George Bush trying to bring more and more power into the executive branch and not go through Congress at all.” I believe he meant it when he cast himself as a principled civil libertarian, when he pledged to defer to Congress on war powers, when he promised to abjure privileges Bush had claimed.

I also believe he was sincere when he told audiences, again and again across his presidency, that a sweeping unilateral move like the one just made on immigration would betray the norms of constitutional government.

So how did we get from there to here? How did the man who was supposed to tame the imperial presidency become, in certain ways, more imperial than his predecessor?The scope of Obama’s moves can be debated, but that basic imperial reality is clear. Even as he has maintained much of the Bush-era national security architecture, this president has been more willing to launch military operations without congressional approval; more willing to trade in assassination and deal death even to American citizens; and more aggressive in his war on leakers, whistle-blowers and journalists.

At the same time, he has been much more aggressive than Bush in his use of executive power to pursue major domestic policy goals . on education, climate change, health care and now most sweepingly on immigration.

Three forces . two external, one internal . might help explain how this transformation happened.

First, public expectations. Across the last century, the presidency’s powers have increased in a symbiosis with changing public expectations about the office. Because Congress is unsexy, frustrating and hard to follow, mass democracy seems to demand a single iconic figure into whom desires and aspirations and hatreds can be poured. And so the modern president, the Cato Institute’s Gene Healy has written, is increasingly seen as “a soul nourisher, a hope giver, a living American talisman against hurricanes, terrorism, economic downturns and spiritual malaise.”And pressure on this talisman to act, even in violation of laws or norms or Burkean traditions, is ever increasing and intense. When presidents aren’t seen as “doing something,” they’re castigated as lame ducks; when they take unilateral action, as we’ve seen in the last week of media coverage, they suddenly seem to get their groove back. And that’s something that even a principled critic of executive power can find ever harder to pass up.

Second, congressional abdication. This is the point that liberals raise, and plausibly, in President Obama’s defense: It isn’t just that he’s been dealing with an opposition party that’s swung to the right; it’s that this opposition doesn’t know its own mind, collectively or sometimes even individually, and so has trouble bargaining or legislating effectively.

This reality has made it harder to cut major bipartisan deals; it’s made it harder to solve problems that crop up within existing law; it’s made it harder for the president to count votes on foreign policy. All of which creates more incentives for presidential unilateralism: In some cases, it seems required to keep the wheels turning; in others, it can be justified as the only way to get the Big Things done.

Which bring us to the third factor in the president’s transformation: his own ambitions. While running for president, Obama famously praised Ronald Reagan for changing “the trajectory of America” in a way that Bill Clinton’s triangulation did not. And it’s his self-image as the liberal Reagan, I suspect, that’s made it psychologically impossible for this president to accept the limits that his two predecessors eventually accepted on their own policy-making ability.

That transformative self-image has shaped his presidency from the beginning: Obama never really looked for domestic issues where he might be willing to do a version of something the other party wanted . as Bush did with education spending and Medicare Part D, and Clinton did with welfare reform. (He’s had a self-admiring willingness to incorporate conservative ideas into essentially liberal proposals, but that’s not really the same thing.)But the liberal Reagan idea has shaped his choices more as it’s become clear that certain major liberal priorities . a big climate-change bill, a comprehensive amnesty . are as out of legislative reach as health care reform proved for Clinton and Social Security reform for Bush. Confronted with those realities, Clinton pivoted and Bush basically gave up. But Obama can’t accept either option, because both seem like betrayals of his promise, his destiny, his image of himself.

And so he has chosen to betray himself in a different way, by becoming the very thing that he once campaigned against: an elected Caesar, a Cheney for liberalism, a president unbound.