There's a moment in every presidential campaign where the most intense political junkies begin to think of the coming November election as an existential question. With the polls showing Donald Trump catching up to Hillary Clinton, we're at that moment now. Political junkies are starting to think that if the election goes the wrong way, the "country is over."

Understandably, many people are nervous that Donald Trump can win. He is a unique candidate. He demonstrates a unique lack of self-control. Some of his supporters have experienced his campaign as a liberation from the social strictures against venting their hatred of other Americans based on their race and ethnicity. Trump and his campaign have amplified racist and anti-Semitic ideas as well.

Yet many Republicans are still going to vote Trump for the most conventional reasons: They're Republicans and he's the party's candidate. They may be pro-life or anti-gun control, and dread Hillary's judicial nominations. They may still think of the GOP as the small-government party. They'll vote Republican because that's what their sort of person is expected to do. Because of this, Trump may win.

If that happens, many of the people who opposed him will feel a sense of panic. What wasn't supposed to have happened will be reality. His trolls will encourage this freakout as well. If this, what else? And what next?

But the reality is there will be constraints on Trump's power and his presidency.

Trump will first be limited by his own character faults, and especially his self-interest. Trump does not have patience. He is distracted by trivial slights. That means he is very unlikely to sweep away the procedural and institutional hurdles to enacting his plans. He won't have the stamina or wiles to launch a popular campaign against the limits on his office. He doesn't have the freakish vengeful stamina that Andrew Jackson had, and even Jackson's presidency basically extinguished itself in his fight with the national bank. A vain and self-interested man can do damage to our governing institutions by prostituting them for his own ends. But the worst dictators tend to be determined ideologues who are willing to pursue utopian vision. Trump is a distracted, needy lech.

There are the formal limits, of course. Namely the states, along with Congress and the courts. Some of the checking power of these institutions is weakened, but not all of it. If Trump wants to make "deals," he is going to need partners.

Trump would immediately present something of a mismatch with the Republican Congress. Whereas Paul Ryan wants to dramatically reform existing entitlement programs, Trump's instinct is to protect or expand them. A Ryan Congress and a Trump White House will have a compelling interest in finding common ground. But Ryan will have political limits as well. George W. Bush proved that an unpopular Republican president can cost his party control of the House, despite the huge advantage Republicans naturally have in off-year elections. Ryan will have an imperative to keep Trump tied within the limits of public opinion.

Where Trump's authority will be harder to constrain is in foreign policy. Really since the Civil War, the president's war-making powers have become less and less constrained over time. In the 20th century, Congress became a kind of as-needed accessory to the president's foreign policy decision-making. Barack Obama has deployed American forces — air support and ground troops — into several civil wars in the Middle East, not only without the permission of Congress, but even against the polled will of the American people. No matter who enters the office of the presidency next, Americans should try to restore the role of Congress and the people in these weighty decisions.

The biggest check on Trump will be the limits imposed on him by the common opinions of the American people. These may seem like they have partially dissolved if he's elected, but they are there. Even if just in polls, a focused and determined resistance to Trump's excesses will be a powerful restraint. At every step where Trump opposes the "establishment's" way of doing things, the media will zealously search for scandal, and they'll surely find it. These will present lots of opportunities for weakening his presidency.

Trump has said he is willing to wear the mantle of anger for his supporters. But if wearing it makes him unpopular, he will quickly look for something more flattering.