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(CNN) —  

It’s way too early to know who is going to win the 2020 presidential race. We don’t know who the Democratic nominee will be, and we don’t know what President Donald Trump’s approval rating will be.

Still, we can look for patterns in the early polling to see what is driving voter preferences. Specifically, are the early patterns for this election looking like 2016 when Trump was able to win a substantial portion of voters who didn’t like him? Or is it more like 2018, when Trump’s approval rating was very predictive of vote choice for House races?

Our CNN poll suggests that, at least against Joe Biden, the Democrats’ best known and most likely nominee, it looks like 2018. That is, feelings about Trump are strongly correlated with their voting preferences.

And that spells potential disaster for Trump.

Voter selection in the Biden/Trump matchup is nearly perfectly predicted by approval of Trump. Among those who approve of Trump, Trump leads 92% to 5%. Among those who disapprove of Trump, Biden is ahead 95% to 3%.

The result of this breakdown is the same as it’s been in pretty much every other poll: Biden currently leads Trump.

That’s because Trump’s approval rating stands at only 44%, compared to a disapproval rating of 53% among voters. To win in 2020, Trump can’t have the election be a referendum on him if his approval rating is this low. He needs to win a substantial share of those who disapprove of him. So far, that’s not happening.

We saw this same paradigm in the 2018 midterms. Democratic House candidates won 90% of those who disapproved of Trump’s job performance, while Republicans took 88% of those who approved. Combining these stats with the fact that Trump’s approval rating (45%) was 9 points below his disapproval rating (55%) meant that Republicans lost control of the House.

To win in 2020, given his current approval rating, Trump needs the voter mindset to be like it was in 2016. In that election, Trump stood at a weak 38% favorable rating to 60% unfavorable rating. But he was able to beat Hillary Clinton because he won 95% of those who held a favorable rating of him, while she took only 77% of those who held an unfavorable view of him.

Why isn’t that same pattern occurring now? One big factor is that, at least against Biden,Trump is not benefiting from an unpopular opponent, like he did in 2016. Biden currently has a favorable rating (51%) that is 10 points ahead of his unfavorable rating (41%).

At the end of the 2016 campaign, Clinton’s favorable rating (43%) at the end of 2016 stood 12 points behind her unfavorable rating (55%). There was a slew of voters (18%) who liked neither Clinton nor Trump, and he won the election thanks to them.

Our poll reinforces a simple notion: Trump will not win in 2020 unless his approval rating jumps or he faces an unpopular opponent, as he did in 2016.

If neither one happens, 2020 will be a referendum on Trump – like 2018 – and he will lose.

These numbers are why it shouldn’t be surprising to see Trump swing at Biden. He’s trying to create an unpopular opponent. We’ll see if that works in the months to come. It could, given that Biden’s favorable rating has already dropped during his re-entry into the political arena.

Of course, even if Biden’s personal ratings decline, it may not be all that good for Trump. Biden has a long way to go before winning the nomination. Democrats are prizing electability more than ever, and, if they sense Biden is weak on that front, they’re likely to choose a different candidate.