Editor’s Note: Julian Zelizer is a history and public affairs professor at Princeton University, editor of “The Presidency of Barack Obama: A First Historical Assessment” and co-host of the “Politics & Polls” podcast. Follow him on Twitter: @julianzelizer. The opinions expressed in this commentary are those of the author. View more opinion articles on CNN.

(CNN) —  

Former President Barack Obama is coming back to the spotlight. In his second major address in the span of one week — the first coming with his powerful eulogy to the late Sen. John McCain that warned of the dangers facing democracy — Obama will launch his midterm campaign tour to help Democrats retake control of the US Congress.

Following a speech at the University of Illinois on Friday, he will take off to Irvine, California, where he will speak at an event for seven Democrats running in Republican districts, and then continue on to other regions.

His reappearance on the public square is an important development at a moment when President Donald Trump’s governing style has generated tremendous concern about the stability of our democratic institutions. While it is true that ex-presidents often remain out of the public limelight, these are not ordinary times.

Bob Woodward’s new book, “Fear,” is filled with stories about top advisers doing everything possible to contain their boss. One stunning moment involves Gary Cohn, the former director of the National Economic Council, literally stealing papers off Trump’s desk because he was so concerned about what the President might do with the information they held. Woodward also recounts people in Trump’s inner circle seriously questioning his intelligence and fitness to serve.

Whether one is concerned about his stability as a leader or the direction of his policies such as immigration, polls are showing that a growing number of Americans are discontented with the way the President is handling his role. In a shocking New York Times op-ed, one anonymous senior official in the administration admits that many fellow officials are “working diligently from within to frustrate parts of his agenda and his worst inclinations … many Trump appointees have vowed to do what we can to preserve our democratic institutions while thwarting Mr. Trump’s more misguided impulses until he is out of office.”

If the stories that are coming out of Washington are true, then it is fair to say that we do have a genuine constitutional crisis on our hands: a President who is not fit to serve and who can’t be trusted to hold the levers of power.

But anonymous op-eds and a bureaucratic “resistance” won’t do anything other than enable President Trump. With congressional Republicans unwilling to serve as a force of institutional restraint, the only real answer would come with a Democratic Congress. Congressional Republicans have been given an opportunity to demonstrate they could prioritize the health of democratic institutions over partisan interests. They have failed that test.

As the conservative pundit George Will wrote in his Washington Post column, referring to the administration’s decision to separate families at the border, “the policy has given independents and temperate Republicans — these are probably expanding and contracting cohorts, respectively — fresh if redundant evidence for the principle by which they should vote. The principle: The congressional Republican caucuses must be substantially reduced.”

This is a message that Obama, more than almost any other Democrat, has the capacity to share with the electorate. He did this at McCain’s memorial and he can do it repeatedly in the next few months. Although Democrats warn that Obama’s re-emergence will energize Republicans to remain steadfast behind Trump, the strength of Republican partisanship does not revolve around the former president. The radicalization of the Republican electorate has been a multidecade process and the extremists in the party find new foils whenever one disappears.

Those Republicans who are willing to come out to vote Republican, despite having Trump in the White House, are already energized. Indeed, this was one of the great lessons of Obama’s term in office: No matter how much Democrats lean over to be civil or to achieve political peace with the other party, the radicalization of the GOP has created an electorate that always sees blood red. Although pre-Trump politics of 2010 and 2014 may now seem benign, Obama underestimated the ferocity of the Republicans in the midterm elections during his presidency. Now he and his fellow party members should not make the same mistake again.

The former president, who remains enormously popular, can avoid speaking about specific policies if he fears that speeches about the Affordable Care Act or the Deferred Action for Childhood Arrivals program might shift the agenda away from the most glaring challenge that the nation faces: President Trump himself.

As a person who held the highest office in the land and handled that role in a manner that earned widespread respect, even from opponents who staunchly disagreed with his ideas, Obama, with his articulate nature, has the gravitas to argue for the fierce urgency of electing a Democratic Congress. Few can forget the emotional power of his address about race relations in Philadelphia during the heat of the 2008 campaign or his powerful farewell address after the tumultuous 2016 election.

Now the nation needs an orator of that potency to make the case that restoring the traditional balance of power might be the most important issue this nation now faces. This needs to motivate every Democrat, and even some Republicans, in addition to fueling a fight for a particular set of policies.

In addition, speaking to America about creating an institutional check on a runaway president, Obama has the capacity to draw crowds and money to the most important races that will determine the future of the House and Senate. While the Koch brothers have the ability to directly deliver big bucks to loyal Republican candidates, Obama has the ability to draw comparable resources simply through his presence in a particular campaign. It is difficult to think of any Democrat at this moment – perhaps except for his wife, Michelle – who can draw the same kind of energy and attention.

Like her husband, Michelle Obama has also joined the effort to bolster Democrats’ chances to win elections in November. She has launched a voter registration initiative, which, while nonpartisan in its messaging, will drive young voters who tend to lean left to the polls.

Is there a danger the Obamas’ presence will somehow crowd out the most exciting voices in the Democratic Party who are starting to prepare for 2020? The answer is no.

If the Obamas handle this the right way, they will only create a stronger platform, which might include a Democratic Congress, from which the emerging stars of the party can launch their campaigns. The notion that a party can’t walk and chew gum at the same time is a figment of a politically insecure mind. Political parties that are really aiming for big victories bring all their assets to the electoral table.

One of the most damaging mistakes that Barack Obama made when he was in the White House was to shy away from the electoral battlefield when the future of Congress and state legislatures was at stake. In my book about his presidency, I have recounted the high costs the Democrats paid by 2016 as Republicans essentially ran the table at all levels of government.

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In 2018, the stakes are even higher. The former president seems to have a clear-eyed understanding of the dangers the nation faces. By taking to the campaign trail to support Democrats across the country, Obama can play a vital role in helping to rebuild the strength of our democratic institutions.