Joe Lockhart is a CNN political analyst. He was the White House press secretary from 1998-2000 in President Bill Clinton's administration. He co-hosts the podcast "Words Matter." The opinions expressed in this commentary are his own. View more opinion at CNN.

(CNN) Sometimes in politics the least surprising development can be the most important.

This is a true thing that has often been lost amid the reality show-style administration of our current President. While it was expected, Barack Obama's endorsement Tuesday of Joe Biden is still critically important to the Democrat's chances of winning back the White House.

Joe Lockhart

Let's start with the raw politics. Former President Obama's nod reinforces Biden's existing strengths among the Democratic constituencies he needs to win the election, particularly black Americans. More importantly, Obama's help on the campaign trail will motivate young people, many of whom have only recent memories of two presidents in their lives.

The contrast between those two -- Donald Trump and Barack Obama is stark, not just on policy but on larger themes like goodness, virtue and purpose. Restoring those things to the White House would banish what Obama called in his endorsement video the "corruption, carelessness, self-dealing, disinformation, ignorance and just plain meanness" of the Trump tenure.

The praise for Biden from Bernie Sanders a day earlier was an additional overt clarion call to America's youth -- that this is their time to have their young voices heard at the ballot box. It was also practical and real help to Biden -- framing his message as something that is progressive, that is bold and does call for structural change where needed. That is a critical narrative for Sanders and Warren voters to accept if Democrats are to succeed in turning them out in the fall.

Read More