• But if we've learned anything from this cycle and from previous elections, the candidates who occupy the top tier of the field aren't set in stone.
  • In the 2008 GOP primary, for example, Rudy Giuliani dominated the rest of the crowded GOP field in the polls while John McCain, the eventual Republican nominee, trailed behind him until January 2008.
  • On the Democratic side, too, Hillary Clinton lead Obama by double-digit margins for months until February of 2008.
  • At this point, it's unlikely that any candidates polling at or below 1% will be the nominee. But given the unpredictable nature of politics, it's too early to declare the 2020 field a three-person race.
  • Visit Business Insider'shomepage for more stories.

On the eve of the third Democratic debate, a number of polls and news headlines are pointing to the bloated Democratic primary narrowing into a three-way race between former Vice President Joe Biden, Sen. Bernie Sanders , and Sen. Elizabeth Warren .

Those three candidates are the only ones who now regularly clear triple-digits in Democratic primary polls and have seen their numbers either stay consistent or rise as other candidates who previously experienced big surges in the polls, like Sen. Kamala Harris , have so far struggled to consolidate support.

But if we've learned anything from this cycle and from previous elections, the candidates who occupy the top tier of the field aren't set in stone.

Read more: Trump advisers are more worried about Elizabeth Warren than any other 2020 Democratic candidate

Previously unknown candidates including Mayor Pete Buttigieg and businessman Andrew Yang have experienced meteoric rises and become breakout stars in the Democratic field, building up dedicated bases of supporters and qualifying for the next rounds of debates.

Meanwhile, the candidacies of politicians initially expected to be Democratic frontrunners like Sen Kirsten Gillibrand , who dropped out of the race after not qualifying for the fall debates failed to gain any traction and have fallen flat.

As the 2020 campaign cycle progresses, the Washington Post has been tracking which candidates led the race in Real Clear Politics' polling average during past presidential primary cycles in this post from Phillip Bump and under the Twitter handle @LedPast.

Both Trump and Biden lead at this point in the 2016 and 2020 primaries, but from very different circumstances

By looking at polling from the 2016 election, it might seem easy to assume that the candidate leading the polls at this point will likely carry the whole thing.

In the 2016 GOP primary, former Florida Governor Jeb Bush narrowly led most primary polls until July and August of 2015, when President Donald Trump took over a commanding lead in Republican primary polls, dominating nearly each one by a comfortable margin all the way through the primary.

Indeed, former second lady Jill Biden argued at the Iowa State Fair that Democratic primary voters should vote for Biden over other candidates who they may like better on the shaky presumption that Biden is the most electable because he leads in the polls.

"You've got to look at the polls ... and if they're consistent and they're consistently saying the same thing, I think you can't dismiss that ..." she said.

While both Trump and Biden lead their respective primary fields at this point in the cycle, their two candidates aren't a perfect comparison, as they entered their respective presidential primaries under very different circumstances.

Read more: Don't buy into the hype that Joe Biden is necessarily the most electable Democrat

Biden's eight years as Obama's vice president and previous 36 years in the US Senate set up Biden to be the establishment favorite and the automatic frontrunner being in the position of defending the Obama legacy and fending off progressive challengers.

Trump, on the other hand, entered the GOP race as the bombastic outsider candidate who promised to take on the establishment which he termed "the swamp" and promising a radical shake-up to the system instead of restoring norms or being the most "electable."

And while Biden's poll numbers have largely remained steady since he got into the race, Trump became the frontrunner not by starting out as the most well-known candidate, but starting out as the underdog and winning over voters with an eye-catching and uniquely populist message. Conversely, Biden is running on a return to normalcy.

Read more : Joe Biden is 2020's 'It's my turn' candidate, but presidential hopefuls who run because they're next in line tend to lose elections

In some ways, Trump's takeover of the 2016 GOP is less comparable to Biden's current lead in the race than it is to the steady surge of Warren who is also running on a message of economic populism in this summer's Democratic primary polls.

GOP debate
GOP debate
REUTERS/Brian Snyder

Previous primaries show just how much poll numbers can change in crowded primaries

While the Iowa caucus is just a few months away, many voters are still undecided at this stage. The state of the race could adjust rapidly as more voters start to pay closer attention and make up their minds.

Take the 2008 election, for example, a race without an incumbent president or a sitting vice president in the running, where both parties held competitive primaries.

In the Republican primary, former New York City mayor and current Trump attorney Rudy Giuliani dominated the rest of the crowded GOP field for months in the polls while John McCain, the eventual Republican nominee, trailed behind far him and several other candidates.

After successfully fending off both Giuliani and former Arkansas Governor Mike Huckabee who also topped a few 2008 primary surveys McCain didn't consistently start leading in the polls until January 2008.

In Real Clear Politics' polling average in early September of 2007, Giuliani led the field with 29% of the vote followed by Fred Thompson with about 21% of the vote, Sen. Mitt Romney in third, and McCain in fourth with 10% of the vote.

And on the Democratic side, Hillary Clinton regularly trounced Barack Obama by double-digit margins in five-way polls well into the winter of 2007, and was perceived as far more likely to beat a Republican nominee than Obama, then a first-term US Senator.

Read more: POWER RANKING: Here's who has the best chance of becoming the 2020 Democratic presidential nominee

At this point in the race, RCP's average showed Clinton with 43% support in Democratic primary polls compared to 23% for Obama.

The tide began to turn around February and March of 2008, when Clinton's lead over Obama steadily narrowed and eventually disappeared. Obama began consistently besting her by margins outside the margin of error throughout that spring, until he won the nomination.

At this stage, it's highly unlikely that the candidates like former Rep. John Delaney , Rep. Tim Ryan , Gov. Steve Bullock , Mayor Bill De Blasio, and Marianne Williamson who hold RCP polling averages of less than 1% and haven't qualified for any fall debates will go on to the win the nomination.

Biden, Sanders, and Warren are in pretty strong positions and will be hard to knock off, but politics is full of uncertainty, and any of them could experience a sudden collapse in the coming months.

Despite the headlines describing the race as now locked into a three-way contest, it's also too early to conclusively rule out any of the mid-tier candidates including Buttigieg, Yang, Harris, Sens. Amy Klobuchar , and Cory Booker , and former Rep. Beto O'Rourke mounting a surprise surge.

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