Sens. Cory Booker (D-N.J.) and Kamala D. Harris (D-Calif.) in September. (Matt McClain/The Post)
Opinion writer

As the Democratic race for the 2020 presidential nomination gets underway, the contenders are trying to determine how to best distinguish themselves. So far, the only unifying theme among all candidates is that President Trump is unfit and he and his Republican policies must go. Trump’s behavior induces extraordinary rage and bitterness that tend to make Democrats want to outdo each other in their desire to claim that the Trump presidency is illegitimate and that they stand opposed to all things Trump. But, starting with the economy, reality appears likely to force Democrats to walk something of a tightrope: How will they be wildly anti-Trump without pledging to do things that would snuff out the robust economy? Is a vote against Trump a vote for fewer jobs and less prosperity?

According to Gallup, “Americans’ optimism about their personal finances has climbed to levels not seen in more than 16 years, with 69% now saying they expect to be financially better off ‘at this time next year.’" As Gallup’s Jim Norman notes, “The 69% saying they expect to be better off is only two percentage points below the all-time high of 71%, recorded in March 1998 at a time when the nation’s economic boom was producing strong economic growth combined with the lowest inflation and unemployment rates in decades.” Additionally, as recent data from the Labor Department shows, wages and salaries for American workers rose 3.1 percent in the fourth quarter of 2018 for the first time in more than 10 years. Democrats can throw around phrases and ideas such as “massive inequality” and “everyone paying their fair share,” but the Trump economic boom is broad and deep. The assorted calls for higher taxes and the socialist patina that Democrats are adopting could produce a referendum on the economy instead of a referendum on Trump’s character.

Given this reality, Democrats obviously will not be helping themselves with half-cocked manifestos such as the Green New Deal that would produce economic ruin. (Disclosure: My firm represents interests in the fossil-fuel industry.) But never mind that. If the economic status quo holds, Democrats in 2020 will mostly have to stick to platitudes and slogans. The Democrats could be in a bind.

And while we are at it, on many vital foreign policy and trade issues, the Democrats will be left with little to say. For instance, no one running for president is going to declare that they will put the U.S. Embassy in Israel back in Tel Aviv. No one running for president is likely to pledge to simply withdraw all the Trump tariffs. Similarly, it is unlikely that any Democratic candidate will pledge that they will put U.S. troops back in Syria or in Afghanistan. And I do not think any Trump rival will suggest that the diplomatic initiatives with North Korea should be shut down. There will never be an end to the debate about immigration, but even there the Democrats are barreling toward an extremist position that looks a lot like open borders.

So, the vain, vindictive “enfant terrible” from our president is doing more than just energizing the Democrats. By fostering bothersome economic prosperity and making foreign policy moves that no one will want to undo, Trump is effectively rendering much of the traditional debate about peace and prosperity off limits to Democrats. He is marginalizing the left’s cast of 2020 hopefuls, mostly limiting what they can discuss to identity politics, grievances, claims of corruption and comical misfires such as the Green New Deal.