Former vice president Joe Biden and Sen. Kirsten Gillibrand (D-N.Y.) confer at a reception for the swearing-in ceremony of Sen. Doug Jones (D-Ala.) in Washington last January. (Melina Mara/The Washington Post)

With the midterm elections complete, the jockeying for the 2020 Democratic presidential nomination will begin in earnest.

Within a few months, maybe less, major contenders will begin officially announcing their candidacies. Already, the national media has devoted an enormous amount of attention to the race, speculating about the future prospects of Sens. Elizabeth Warren (D-Mass.) and Bernie Sanders (I-Vt.), as well as former vice president Joe Biden. Some even believe that former nominees Hillary Clinton or John F. Kerry will roll the dice again or that Michael Bloomberg might leverage his billions to make a bid.

What do all of these potential candidates have in common? They would all be over the age of 70 if nominated, with several pushing 80. And this is a problem for the party.

Historically, older Democrats have faced significant challenges when seeking the highest office in the land. They have found it difficult to galvanize progressive voters, who are attracted to fresh policy ideas and bold thinking. They also have sometimes separated the party from young voters, one of its core constituencies over the past century. As a result, activists and donors may want to focus most of their attention on the slew of younger aspirants considering a run for the White House, even if they seem like long shots two years before the election.

On the surface, it may seem that the age of major-party candidates has little influence on who wins the presidency. Since the 1932 election, the average age of presidential nominees, both winners and losers, has been 57 years old. If anything, there is a slight indication that it is better to select older candidates. In the 22 elections contested during this period, the older nominee has won 13 times.

But these numbers don’t tell the whole story. In fact, there is a deep connection between party ideology and the selection process that makes age matter more for Democrats than it does for Republicans.

During the period from 1860 to 1928, Republicans were the more progressive party in American politics. In that era, the average Republican nominee was 53 years old — while the average winner was a year younger. During the mid-20th century, however, the GOP made its fateful shift toward conservatism. As a result, Republican voters came to revere tradition, patriotism, individualism and, most importantly, hierarchy. They have in the decades since increasingly opted to support older presidential candidates.

Over the past four decades, this shift became even more pronounced. Since 1980, Republicans have nominated a candidate under the age of 60 only once. The average age of nominees rose to 66.

The Democratic Party also flipped during the early 20th century, moving toward progressivism, with the shift cemented by the success of Franklin D. Roosevelt. As a result, the party’s voters came to adopt a belief in positive rights, holding that government has a duty to protect the general welfare through social insurance programs. This new outlook resulted in Democrats steadily favoring younger nominees. The average Democratic presidential winner was a full decade younger than the average Republican winner, 53 versus 63.

The numbers are even more striking when looking at Democratic non-incumbents (Roosevelt, John F. Kennedy, Jimmy Carter, Bill Clinton and Barack Obama) who triumphed over the last century. One of the reasons the average age of nominees since 1932 remains relatively high is because so many incumbents won reelection. Non-incumbents' average age, however, was only 48 — a full 14 years younger than their Republican opponents.

In fact, in the last century, the party has never won with a non-incumbent who was older than 52. Hubert Humphrey, Walter Mondale and Michael Dukakis were all veteran, accomplished politicians in their mid-50s, while Kerry and Hillary Clinton were both over 60. All lost.

The youth of successful Democrats makes perfect sense. Democrats are the party of progress, of hope and of change. It seems quite sensible, therefore, that their voters would gravitate toward youth, vigor and new ways of thinking. In short, the progressive coalition seeks candidates who represent how they see themselves. And these younger nominees have been able to successfully bridge reform impulses across generations. Roosevelt (age 50) united the Missionary Generation that had backed Woodrow Wilson’s progressivism two decades earlier and the GI Generation by promising programs that would provide economic security. Kennedy (age 43) joined the GI Generation and baby boomers around the idea of further expanding social insurance programs. Bill Clinton (age 46), in turn, animated baby boomers and Generation Xers with an agenda focused on restoring fairness to the economy. Finally, Obama (age 47) fused Gen Xers, millennials and nonwhite voters around a message concentrated on reforming the health-care system and working toward energy independence.

Warren, Sanders, Biden, Hillary Clinton, Kerry and Bloomberg would all be more than two decades older than the oldest non-incumbent Democrat elected to the White House in a century. During that time, the Democrats have elected only two people over 60 to the presidency: Roosevelt for his fourth term, and Harry Truman when he won election after ascending to the presidency upon Roosevelt’s death.

Today, Generation X and the millennials have become a majority of the electorate, and more than a quarter of voters are nonwhite. And so it seems prudent that activists and donors should be critically assessing the crop of younger hopefuls who would seem to better fit the mold for successful Democratic contenders. The potential candidacies of people like Cory Booker (49), Julián Castro (44), Eric Garcetti (47), Kirsten Gillibrand (51), Chris Murphy (45) and Beto O’Rourke (46) probably warrant much closer inspection based on these historical trends. It might even be worth discussing still younger potential nominees, such as Tulsi Gabbard (37), Joseph P. Kennedy III (38) and Seth Moulton (40), as long-shot entries who may be more suitable for the national mood.

Democrats need to recognize that in 2020 they will be facing the oldest incumbent president in U.S. history. In many ways, it will be an election of juxtapositions. In what they hope will be a change election, perhaps the most powerful distinction progressives can make would come from nominating a youthful candidate — one who speaks to America’s future and stands in stark contrast with a president whose focus is restoring the past.