Dean Obeidallah, a former attorney, is the host of SiriusXM radio's daily program "The Dean Obeidallah Show" and a columnist for The Daily Beast. Follow him @DeanObeidallah. The opinions expressed in this commentary are his own. View more opinion articles on CNN.

(CNN) President Richard Nixon, in the middle of the Watergate scandal, famously said, "People have got to know whether or not their President is a crook." In the case of Donald Trump, America has already weighed in on that issue. According to a recent Quinnipiac poll, "64% of voters said Trump had committed crimes before entering office." This is particularly astounding, given how difficult it is, in our hyperpartisan nation, to get nearly two-thirds of voters to agree on anything! (For the record, Trump denies any wrongdoing.)

Dean Obeidallah

And sure, it would be tempting for the 2020 Democratic presidential candidates to capitalize on this, and make Trump's past actions (and tweets) their No. 1 issue during the Democratic primary contest. But they shouldn't.

By now, American voters are very familiar with who Trump is. There's a reason why Trump has the highest disapproval rating of any modern president: almost 53% . Even Jimmy Carter, who lost re-election, only had a 39.3% disapproval rating at this point.

Trump is flawed, but it's not enough for Democratic presidential candidates to point at that. What voters need to hear from them is how they'll be different from him -- where do they stand on the critical issues facing our country? And this is precisely where the candidates should focus their time, both in the primary and in the general election.

In short, they need to give voters a reason to come out and vote for them.