Sanders, Trump won't be able to keep their promises. And that matters.

Even before Iowa, it was apparent that outsiders had a very strong appeal and that their promises were resonating.

Iowa has simply added an exclamation mark.

Voters frustrated with illegal immigration are pinning their hopes on the Great Wall of Trump. If they’re Bernie Sanders fans, they’re looking forward to the breakup of the big banks, Medicare for all, and free tuition at all public colleges and universities. 

In truth, none of this is going to happen, no matter who is elected to the White House. That’s not because these candidates, once in the White House, will turn into a squish. It’s because of the way the Founding Fathers designed the United States government – to take baby steps, not giant leaps.

The irony of the outsider campaigns is that such promises are not the cure for the disillusionment of large swaths of the American electorate, but in some ways the cause of it, experts say.

Bold promises are part and parcel of presidential campaigns, of course. Voters like strong rhetoric and reward it. But this year’s promises by the antiestablishment candidates are “especially sweeping,” driven by a desire to tap into the angry voter, says Amy Black, a political scientist at Wheaton College in Illinois. 

“Outsider candidates are responding to voter anger with promises they have no chance of keeping,” she writes in an e-mail.

The concern is that this cycle could only stoke voters’ frustrations further – especially if one of the outsider candidates wins. 

“President Trump is not going to be able to build a wall and make Mexico pay for it. President Sanders is not going to be able to get ‘single-payer’ health care,” says John Pitney, a political scientist at Claremont McKenna College in California. “I don’t know that [such voter] disillusionment is going to be very healthy for political participation in the future.”

The simple truth is that the American system was designed to be slow and inefficient, says Professor Black. The Founders feared sweeping changes and a federal government that could respond too quickly to popular whims. So they set up the three branches, two chambers, and federalist system to accommodate the country’s vast diversity of opinion and geography and prevent too much power from accumulating anywhere.

For those disappointed that their candidate won’t be able to go in and just make things happen, the comfort – and the protection – is that the other guy’s candidate won’t be able to do so, either.

“The system that frustrates one’s own policies is also the system that frustrates the opposition’s,” says Professor Pitney.

Take the border wall.

No president can simply, on his own, build a wall that stretches across the US border with Mexico, as Mr. Trump and Sen. Ted Cruz have proposed. It needs to be paid for, and Congress controls the purse strings. Even if Republicans swept to victories in both chambers of Congress, they would need a 60-vote majority in the Senate. And even if they had that, some Republicans might think twice before taking a step that would deeply anger Latinos, a growing voting bloc.

Likewise, replacing the Affordable Care Act with a single-payer plan or breaking up the big banks, as Senator Sanders proposes, would face great hostility in Congress. For starters, the plans would have to get through the House, which the GOP is expected to retain. 

That’s not to say America hasn’t had its share of bold presidents taking bold action. But it usually takes a “very clear threat to the United States, either externally or internally” to allow such boldness, says James Thurber, director of congressional and presidential studies at American University in Washington.

He points to threats such as the breakup of the nation, in President Lincoln’s time, or the Great Depression and Pearl Harbor, under President Franklin Roosevelt. Voters have to generally agree on what the problem is, and in the case of immigration and health care, they don't, he says.

Then there is the popular mandate. Democratic President Lyndon Johnson’s landslide victory over Barry Goldwater in 1964 – and earlier, President Kennedy's assassination – gave him the support he needed for his civil rights, voting rights, and antipoverty initiatives.

Even so, the opportunity for a transformative presidency is brief – circumstances change and so do voters. Even Johnson realized this, says Pitney.

By design, America’s government requires a certain amount of consensus to get things done. And some candidates – from Hillary Clinton to Jeb Bush and John Kasich – are campaigning in that spirit, proposing policies that could actually stand a chance of navigating the constitutional checks and balances.

But in Iowa – with the exception of Mrs. Clinton, who squeaked out the narrowest victory in Democratic caucus history – they were not the ones with the energy and momentum. And that, in itself, is telling.