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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 —  

Sens. Elizabeth Warren, Kamala Harris and Cory Booker were among the potential 2020 presidential candidates who spoke this week about where the party needs to go at the Netroots Nation conference in New Orleans, adding to the internal debate on the left about strategy. Yet, there should be no mystery about the right answer.

Democrats need to stop driving themselves crazy about an “identity crisis” that is supposedly bogging down the party as it heads into the height of the midterm campaign season. With ongoing debates about what the party should stand for – the Alexandria Ocasio-Cortez wing or the Conor Lamb wing – the sole focus for every Democratic candidate should be on the giant elephant on the campaign trail: President Donald J. Trump.

With a commander in chief who is as divisive and polarizing as our current President, Democrats already have the issue around which every candidate and voter can rally – namely the need to stop President Trump and his rightward agenda.

Democrats should forget about writing a new “Contract with America” or finding a perfect set of issues that can unite every Democrat in the country. That won’t happen; it rarely does under any circumstances. What Democrats need are coalitional themes, the way that Republicans used President Bill Clinton’s failed health care program in 1994 or Democrats used the war in Iraq in 2006 or President Trump is currently using so-called “fake news” and the supposed “witch hunt” in his rallies to bring the different GOP factions together.

The theme needs to be stopping – but not removing – Trump. It’s that simple. For the time being, Democrats need to forget impeachment, forget resignation, forget the 25th Amendment, and forget about any other quick process that would theoretically change the current reality of Washington. The likelihood is that Trump will remain in the White House – and talk of ousting him is certain to energize his base to turn out to vote for the GOP in November.

But a win in November would give Democrats the tools they need to oppose his policies.

The best and most effective starting point to launch a new political era would be to elect a Democratic Congress. Few things work as well in American politics as a genuine check and balance.

But Democrats need to stand against President Trump in the right way. If the next few months turn into a series of ongoing campaigns that revolve around hating the man – the kind of pure anger one often sees among liberals on the television screen – the plan will backfire.

The temptation to rage against the machine is strong, given that the President offers more material almost every hour. If that is the tenor of the campaigns, however, Democrats could diminish their chances for achieving the so-called wave election that the party is desperately hoping for.

The key will be to connect the President to a series of hot-button issues on which Trump embodies policies and values that don’t command the support of the majority of this nation. In an election cycle during which purple districts and states might play a pivotal role in the future of Congress, such a strategy could be extremely effective.

The Democrats need to turn 2018 into a values election. As voters enter the ballot box, they need to make a decision about what values they want this country to stand for.

What would that message look like? For one, voting against President Trump is a vote in favor of protecting our environment. According to a recent Gallup poll, 62% believe that the government is not doing enough to protect the environment.

President Trump’s administration has worked diligently to roll back as many regulations as possible before his time his done. Now, the EPA is under the leadership of Andrew Wheeler, who took over from the infamous Scott Pruitt and who promises to be even more effective at moving forward with this agenda, given that he is less interested in living large and more savvy in the ways of Washington.

Voting against President Trump is a vote in favor of real economic populism. Even though the economy is roaring, many Americans continue to struggle to make ends meet and worry about the future of their children.

While President Trump’s rhetoric has been all about helping the forgotten American, the core of his policies, such as the corporate tax cut and a potential unilateral capital gains reduction that would hugely benefit wealthy Americans, does nothing to help working families. His trade war has heightened the economic challenges that Americans working in many industries face, including so many people in the rural communities that voted for Trump. Some Democrats have shrewdly been talking about issues such as protecting Americans with pre-existing health conditions from higher insurance costs as a way to make this very point.

Voting against President Trump is a vote in favor of common sense gun regulation, which a majority of Americans also support. Ever since the protests following the shootings in Parkland, Florida, the inspiration generated from the kids fighting for safe schools has been counteracted by disappointment that Congress has done nothing.

Once again, a large majority of Americans want stricter gun regulations.

Voting against President Trump is a vote in favor of the pluralism and social diversity that characterizes the United States in 2018. Consistently, President Trump has made provocative statements and taken stands, particularly with regard to immigrants, that go against the intricate social fabric that defines most of our communities.

On the eve of the anniversary of the violent Charlottesville, Virginia protests, after which Trump refused to come down hard on neo-Nazis, Democrats should be doubly energized to make this point.

The #MeToo movement is one of the most powerful grass-roots forces in American politics today. Educated suburban female voters are organized and mobilized for this election, and this is a theme that Democrats must make one of their highest priorities. As Kamala Harris told Netroots Nation, there is no need to listen to the critics of “identity politics” who are trying to “shut us up.” Civil, immigrant, and women’s rights “will define our identity as Americans. This is about American identity.”

Voting against President Trump is a vote in favor of respecting and cherishing the political institutions that this nation has been building, and rebuilding, since the founding of the nation. If there is any issue on which President Trump is vulnerable among some Republican voters and certainly independents, it is the way he ignores, attacks, and dismisses our institutions of governance – from the presidency to law enforcement to the media.

Although many Americans keep thinking he won’t go there in his attacks on the legitimacy of these institutions, he keeps going there. Watching the President issue his standard talking points while standing next to President Vladimir Putin in Russia was shocking for this very reason.

In this regard, Democrats can paint themselves as the conservatives, as the protectors of tradition and institutions. With a radical like President Trump in the White House, they can turn around the familiar narrative whereby Republicans paint them as extremist agents of instability on its head.

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There are more than enough issues on the table for Democrats to roll the GOP in November if they can organize, mobilize, and get the right message out to the electorate. The sharp rightward shift in the Supreme Court offers one of the most dramatic manifestations of what will continue to happen should there be no congressional check on the Trump administration.

Democrats don’t need to worry about division and factions. They need to focus all on their attention on making this election a referendum on President Trump and explaining to voters that taking a stand for a Democratic Congress is the only way to move the country forward on a series of key policy issues that concern a great majority of the nation.