(CNN) Whoever wins the US presidential election in about 18 months and how they do it will depend on a bunch of different factors, but probably the most important is the one President Donald Trump's acting chief of staff laid out Tuesday.

"You hate to sound like a cliche, but are you better off than you were four years ago? It's pretty simple, right? 'It's the economy, stupid.' I think that's easy," Mick Mulvaney told an audience at the Milken Conference in Los Angeles. "People will vote for somebody they don't like if they think it's good for them."

That backhanded endorsement summarizes the challenge Democrats face. Their vast field of candidates offers everything from transformational democratic socialism to a return to normalcy. And we haven't yet gotten to the portion of the primary season where they point out their own differences and get serious about making their pitches on who should win the right to face Trump in 2020.

But it's a matter of fact that some Democrat will face Trump and after that, there will be a Democratic president or, barring something completely unforeseen, there will be Trump.

And the reality is, voters don't like to punish incumbents when the economy feels fine. Remove Gerald Ford, who was never elected in his own right and had just pardoned Richard Nixon, and you have only three incumbents who've lost reelection in the last 100 years.

The other three each had an economic calamity they were dealing with.

Even Ford's loss in 1976 was in part thanks to stagflation and the weak recovery from the energy crisis

What's Trump got? He's overseeing a strong economy, near-low unemployment, unexpectedly strong growth in the gross domestic product, a surprising lack of consequences for the trade war he's tempted with China and Europe, and a strong dollar. It's been nearly 10 years of nonstop growth for the US economy, an expansion that will set a record starting in July

In March, 71% of Americans said the economy was in good shape and a majority, 51%, approved of Trump's handling of it. That his overall approval rating remains well under 50% despite this economy will test the second part of what Mulvaney said -- that people will vote for somebody they don't like if they think it's good for them.

Trump has already proved he can get people who don't like him to vote for him. In 2016, according to exit polls, 60% of voters had an unfavorable view of Trump. Fifteen percent of those voters chose him anyway

Not all Americans feel like the economy is working for them. Most voters (60%) think the economic system benefits those in power rather than all people, according to an ABC News/Washington Post poll out Monday, although less than a third of Republicans (32%) said the economy benefits those in power as opposed to all people. Strong majorities of Democrats and independents said the opposite.

A larger majority of all voters (72%) said the US system of government benefits those in power.

In a separate poll, conducted by Monmouth University , just 12% of Americans said their families have benefited a great deal from the current economy and 31% said they had gotten some benefit. A combined 54% said they had been helped not much or not at all.

That should signal an opening to Democrats.

In the populist pitches from Democratic presidential candidates like Bernie Sanders and Elizabeth Warren, senators from Vermont and Massachusetts, is a promise to give more Americans the economic security already felt by the wealthy. They want to divert American wealth that already exists to providing better health care coverage for everyone, college tuition for everyone and, in Warren's case, child care for everyone.

But these are transformational ideas that skirt Mulvaney's question. They're asking voters to creatively think about a different United States.

More conventional is Joe Biden, the Democratic front-runner. He's certainly arguing to Americans that they are worse off under Trump, but for moral reasons, not economic ones. He made his first pitch as a candidate arguing to voters in an internet video that he would be the antidote to Trump's incendiary speech and commiseration with white nationalists, not that he would make voters' lives materially better.

That's a message that will resonate with the many voters who dislike Trump -- 92% of Democratic and Democratic-leaning voters said in a new CNN poll conducted by SSRS that selecting a candidate who has a good chance of defeating Trump is important to their Democratic primary decisions. It might not resonate as much with those whose focus is on their daily lives, not politics.

And according to Mulvaney, Trump's pitch is going to stay that simple.

"We're talking about the economy, we're talking about health care, we're talking about trade and we think we've got a very compelling argument heading into 2020. We also think we have a lot of really weak competitors, which is fun," Mulvaney said.

We'll have to see how much Trump's Twitter feed gets in the way of that messaging. Otherwise, it could be a compelling argument for the President, who rode the economic recovery overseen by his predecessor into the White House.

Frustration among voters in the Rust Belt with the collapse of manufacturing and other working-class trade -- among other things -- helped Trump win in 2016. Fear of economic apocalypse and frustration with bailouts for big banks played a bigger role than people might now remember in Barack Obama's victory over John McCain in 2008. The dot-com bubble that went poof in the late 1990s made voters hesitant about Al Gore, who had bragged about taking initiative in creating the internet. Did that help George W. Bush squeak into the White House? Sure. Bill Clinton, clearly, preyed on the economy as his top issue in 1992.