Recent polls appear to give Democrats running for office in 2020 comfort, but in reality, it should cause concern. The leads they enjoy are not as great as they appear, because leads 16 months ahead of presidential elections are tenuous, particularly against incumbents. In this campaign, they are likely most particularly so.

Polls aplenty show Trump in apparent trouble. NBC/Wall Street Journal's latest poll shows Biden leading 51% to 42% and Trump trailing Kamala Harris (by two points), Warren (five points), and Sanders (six points). Yet in presidential elections, incumbents are remarkably resilient and challengers extremely evanescent.

In the last 100 years, only three elected presidents lost reelection: Herbert Hoover, Jimmy Carter, and George H.W. Bush. All the rest won, and all except Obama in 2012 increased their popular vote percentage the second time around. Even Obama is still winning a popular vote majority.

Incumbents do so well because they transform in office. Rarely does either party hold the presidency more than two consecutive terms, so most presidents initially win by challenging the status quo. However, after four years in office, these former disruptors become the status quo. If times are good, as they usually are, Americans stick with them.

Such incumbents also gain enormous advantage from controlling the political agenda. The presidency has grown for roughly a century. Its ascendancy gives incumbents huge opportunities no challenger can match.

On this tilted field, challengers usually suffer sliding support at the same time that incumbents increase theirs.

Challengers start filled with promise (figuratively and literally) and potential. Comparative unknowns, they exit the gate quickly. However, campaigning realities and rigors — usually contesting contentious primaries that incumbents avoid — are much less kind.

Running against the status quo, they must convince voters to chance it on the promise of something better which is usually a tough sell. Unable to control the political agenda, they must be reactive which is an increasingly difficult proposition with today’s continuous media coverage.

Additionally, challengers are more susceptible to “surprises.” Enduring microscopic coverage for four years, incumbents receive a certain inoculation: There is little new to uncover.

For challengers, the opposite prevails. However much scrutiny public figures have had, nothing compares with what they receive as a presidential nominee. Having proactively publicized all the good about themselves, challengers have only a downside from new revelations. And having never been so thoroughly covered, there is voracious demand for something new.

Facing this general pro-incumbent bias, the Democrats' nominee faces an even tougher 2020 task. On the economy and the public's dominant measure of the status quo, Trump has a decided advantage. The three elected presidents who lost reelection all had weak economies. And Trump’s is a twofold advantage: His economy is both strong and decidedly stronger than Obama’s.

Regarding controlling the political agenda, while any incumbent president can, few have to the extent Trump has. That will not change once the campaign really starts: If anything, it likely increases.

Any 2020 Democrat will face unique difficulties as a challenger. The biggest will be their base’s requirement that their nominee run from the Left. That means starting from America’s smallest ideological group. To win, they will need support from moderates and conservatives.

That will be particularly tough in 2020. The Democrats’ nominee will not just be coming from the Left, but the hard left. The Democrats’ primary field is extremely crowded with almost all on their spectrum’s left, and their spectrum is left to begin with. Competing in this shared space, differentiation will require “out-lefting” their competition and taking them further from the needed center.

Proportional awarding of delegates ensures the Left’s candidates will have time to stay in the race and eventually consolidate into one. At the same time, Biden, the Party’s most establishment candidate, is weak, posing little chance of a quick knockout.

On the “surprise” side, the Democrats’ nominee will face the usual heightened risk, even given favorable media coverage. As for Trump, he has been inoculated by consistently brutal media coverage, to the point that he can credibly claim to be a victim. With everything already thrown at him, the backfire is real with his base, and at time will even reach those outside his base.

If you want to beat an incumbent with a strong economy, you need a substantial lead to survive a presidential onslaught. Democrats challenging Trump from the extreme left will particularly need it. It is far from clear that Democrats have enough now and even less clear that they can hold it.

J.T. Young served under President George W. Bush as the director of communications in the Office of Management and Budget and as deputy assistant secretary in legislative affairs for tax and budget at the Treasury Department. He served as a congressional staffer from 1987 through 2000.