Over at Polyarchy, Vox's other independent blog, Lee Drutman has a clever and systematically thought-out piece on the possible outcomes of a Trump presidency. My specific quibbles with his speculations about Trump are limited, but his analysis has prodded me to write something I've been thinking about quite a bit lately. A dominant theme in political science blogging is the mockery of journalists and others who've bought into the "Green Lantern presidency" myth and who think that presidents can fundamentally alter the political landscape just by doing a good job "leading," whatever that means.

This isn't wrong, but some of the extensions of this idea miss the mark: The American presidency is not omnipotent or magical. But it is very, very powerful.

Let's start with some quick acknowledgment about what the president can't do: He can't make landmark legislation happen through sheer force of will, rhetorical brilliance, or public tours (as Woodrow Wilson did to promote the League of Nations or George W. Bush to promote Social Security reform). He can't change people's minds by giving speeches. Most people either don't care and aren't listening, or have already made up their minds. He can't, as Barack Obama has found again and again, change how politics works, cleanse the system of its dependence on "special interests" (a.k.a. the First Amendment), or achieve unity between the two parties. In other words, presidents can't do some of the things we might most visibly be able to associate with leadership. They can't make people do things they don't want to do, or dislodge entrenched aspects of the system. But presidents still occupy a unique place in the system. They are part of it — they lead the executive branch, where a lot of power resides, much of it bureaucratic and unsexy but nevertheless consequential.

Policy really comes to life in its enforcement. Presidents can use this process in a variety of ways to infuse their own priorities — see executive orders on the "global gag rule" about abortion counseling, for example. Presidents can also empower or strangle agencies, depending on what they think of the mission. Civil rights enforcement has contracted under recent Republican administrations, as has enforcement of environmental regulation. And Obama's presidency has provided plenty of examples of selective enforcement to support liberal priorities, such as executive action on deporting undocumented immigrants.

Second, although presidents cannot make members of Congress do anything they don't want to do, they can exercise considerable leadership and influence over the agenda. Obama's pursuit of health care reform is a good example of this — it's entirely possible that Democratic members of Congress would have preferred to start with a broader economic bill or environmental issues or something along those lines. Presidents can set the agenda based on personal conviction — but they're also in a position to look at the big picture of what party members might be prepared to support, or what might fit a national party agenda, while members of Congress have historically had more narrow political incentives. A misfire in this regard can make a difference, too. What if Bush had pursued something other than Social Security reform in 2005?

In addition, presidents, as political theorist Harvey Mansfield famously noted in Taming the Prince, are charged with the necessary discretion to defend and preserve the Constitution, to take care that the laws are faithfully executed — a power that can be read as minimal and even as the president being a "clerk" of Congress — or as a power that, as Mansfield observed, is impossible to fully subsume under the Constitution.

This really makes a difference when it comes to national response to unexpected, unprecedented, and complex situations. Think the Civil War, the Cuban missile crisis, 9/11. These kinds of events are, of course, rare. But the two fairly marginal powers listed above — enforcement and agenda setting — become central in a crisis. The results can be lasting and far-reaching. And the structure of the presidency, set up to enable secrecy and immediate action, makes it very difficult to confine. It's true, for example, that Bush's decisions in the Iraq War eventually became unpopular, resulting in Democratic victories in 2006 and a contentious divided government for Bush's last two years. But nevertheless, we are still contending with the impact of those early decisions.

The rare leader who isn't constrained by party ties may warrant particular caution. Most of our presidents, like Bush, have been creatures of party, almost by definition. But we've had a few accidental presidents who existed outside those structures: John Tyler, and Andrew Johnson, appointed to a unity ticket with Lincoln in 1864. Johnson, as president, got first crack at what Reconstruction in the post–Civil War South would look like. He also vetoed several bills to benefit freed slaves and establish a rights regime in the wake of the 13th Amendment, drawing on justifications that foreshadow later racialized arguments about welfare. His decisions set the stage for party infighting and decades of further oppression for African Americans in the South. Drutman's piece about limits on the presidency mentions checks and balances — parties are an important, if less formal, check on how presidents conduct themselves in office. Johnson, a Democrat leading a Republican administration, was not so encumbered.

One really unpleasant truth that emerges from this is that there's a certain asymmetry to what presidents can do. A really skilled and brilliant president can't fix everything. But one with bad judgment can do lasting damage.