How to read 2020 polling right now

Democratic presidential nominee Barack Obama and Hillary Clinton attend a campaign rally together on October 20, 2008, in Orlando, Florida.

(CNN)It might not even be 2019, but political pundits are already hyper-focused on 2020. And with good reason. If history is any guide, the next Democratic nominee is most likely already in the planning phase of a 2020 campaign and will start running in earnest early next year. And he or she likely already figures prominently in polls of primary fields like the one CNN conducted last week.

Election polling and precedent aren't perfect predictors, though. At this point in 2014, when 2016 primary polling was gearing up, Donald Trump hadn't declared himself as running for president and wasn't being included in any primary surveys. However, that is the only example of a time that the ultimate nominee for a main party hasn't been in the top five of early polls taken since 1996. In fact, every cycle except for the 2016 Republican primary and the 2004 Democratic primary had the eventual party nominee in the top three for their party at this point.
To put it another way, in open primaries where there is no incumbent since 1996, the candidate who went on to win their primary was in the top three of an aggregate of polls taken between November (post-election) and January (two years before the election) 7 out of 9 times.
One huge caveat is that there was an open field for Democrats in 1992, but not enough early primary polls to create an average. In the one poll conducted by Gallup, 18 candidates were included, but not ultimate winner Bill Clinton. None of the candidates who were polled received above 7% support and the majority of voters said they weren't sure yet or didn't want to vote for any of the choices given.
    Hillary Clinton was polled in the 2000, 2008 and 2016 elections. In November and December of 2014 and January of 2015, nine polls were taken of the Democratic primary field. In every single poll, Clinton was the number one choice for likely Democratic voters. She had an average of 60% support in all the polls, followed (not closely) by Elizabeth Warren's average of 10%. Warren did not end up launching a campaign, and Clinton was the first Democrat to officially declare candidacy in April 2015.
    The Democratic and Republican primaries in 2000, as well as the Republican primary in 1996, had the respective winner in first place in every poll taken from November through January. While ultimate GOP nominee Mitt Romney didn't gain first place in every single early poll, his average from post-2010 election through January 2011 topped Mike Huckabee's by 1 percentage point.
    In 2008, which saw an open primary fight for both parties, ultimate GOP nominee John McCain and ultimate Democratic nominee Barack Obama both averaged second place in these early polls. Though Hillary Clinton dominated Obama in most early polls, he consistently stayed behind her until eventually taking the lead in more than a few polls in February 2008.
    An exception to the rule was in the 2016 Republican primary, when Trump wasn't considered a serious candidate by most pollsters and media outlets until June 2015, when he announced his campaign. He was the top Republican in polls within a month of declaring his candidacy.
    A fun fact, though, is that the first time Trump was asked about as a contender for a presidential election was in July 1999, by Fox News as a third-party candidate in the race between Al Gore and George W. Bush. Trump launched an exploratory committee to consider a Reform Party run but never formally launched a campaign. Only 6% said they'd support Trump over Bush (53%) or Gore (31%), the eventual nominees.
    A second exception was in the 2004 Democratic primary. Although John Kerry was eventually nominated to face off against Bush in his bid for re-election, the average of Kerry's polls between November 2002 and January 2003 was around 13%, number four in the field, behind Gore, Clinton and Sen. Joe Lieberman of Connecticut. Gore, who had the highest average in November and the first half of December, declared in mid-December 2002 that he wouldn't run for president. Gore's average is, therefore, only factoring in a month of polling (which was very good for him). While Kerry remained lower, he was in the top three by January.
      Of course, polling isn't meant to be predictive. Polls are a snapshot of what is happening at that point in time, not a prognosis for what will happen. That's what forecast modeling is for.
      So what does this mean for 2020? In a new CNN poll conducted by SSRS, Joe Biden led the pack, with 30% of the likely Democratic voters. Following were Sen. Bernie Sanders of Vermont (14%), Rep. Beto O'Rourke of Texas (9%), Sen. Cory Booker of New Jersey (5%) and Sen. Kamala Harris of California (4%). If the 2018 Democratic primary follows past patterns, one of those five could be the nominee to go up against the Republican competitor (likely Trump). So far, there have been few polls conducted on the 2020 nomination after the midterm election, fewer than recent elections. But it's likely that more will come with the new year.