By Julian Zelizer, CNN/StyleMagazine.com Newswire

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.