(CNN) The 2020 Democratic presidential contenders are already discussing the issues that will help to determine who wins the party's nomination. But rather than one policy issue such as health care or foreign policy, polling is making it clear that something else will be key to success in the primary: President Donald Trump.

The President will shape the Democratic presidential primary in a way not seen in any recent year.

In late January, 43% of Democratic respondents said in an ABC News/Washington Post poll that it was more important to them to choose a candidate who seemed most likely to defeat Trump than it was to choose a candidate closest to them on the issues. That was only slightly behind the 47% who thought that issues were more important.

In a differently worded question, Monmouth University found this week that 56% of Democrats said they rather have a candidate who they don't agree on most issues but was stronger against Trump than the 33% who said they wanted a candidate they agreed with but would have a hard time beating Trump.

These polls followed a CNN Iowa poll in which 54% of Iowa Democrats said beating Trump was more important than a candidate who shared their position on major issues. Just 41% said issues were more important to them.

I don't pretend to know which candidates Democratic voters think will have the best chance of beating Trump. The default answer is probably someone who has done well in the past or is closer to the political center. Indeed, that's another reason to believe that there is room for a more moderate Democratic contender than is currently running in the primary.

In reality though, someone could make the case that they are more electable because they can draw the sharpest ideological distinction with Trump, can raise minority turnout or can even make the case that they are an independent (i.e. Bernie Sanders).

What is not up for debate is the unusually high percentage of Democrats who are prioritizing winning the general election than finding a candidate who agrees with them on the issues.

From 2004 to 2016, CBS News has been asking voters a very similar question to what ABC News/Washington Post asked nationally and what CNN asked in Iowa. (What is more important: "nominee who agrees with your position on most issues, or a nominee who can win the general election in November?")

The highest percentage recorded at this point in any of those six contested primaries for choosing a nominee who could win in November was 35%. On average, it was less than 30%. This year, all polls find that greater than 40% of voters say beating Trump is more important. Meanwhile, the percentage of voters who said a candidate's position on the issues was most important never fell below 60% and averaged 66% from 2004 to 2016. This year, less than 50% of Democrats say it is most important to nominate a candidate who is closest to them on the issues.

It really shouldn't be surprising that Trump will play a historically important role in the 2020 Democratic primary. The strength at which Democrats oppose Trump is unusually high. It is as high as it was against former President Richard Nixon before he resigned in 1974. Opposition to Trump was a bigger factor in voters' 2018 midterm preferences than opposition to any incumbent president has been in any midterm in the last 40 years.

The bottom line is that Trump has dominated American politics over the last 3 and a half years. We shouldn't expect that to change this primary season. His role even in the opposition's primary will be huge for voters. If Democratic candidates aren't able to sell Democratic voters that they can beat Trump, it won't matter how strong their policy chops are.