(CNN) No one knows with any real certainty who will win the 2020 Democratic presidential primary. One decent bet to make is that former Vice President Joe Biden and Vermont Sen. Bernie Sanders are the most likely candidates to hold onto to a firm base of support as we head into the primary season.

Over and over again, Biden and Sanders' voters say they are the least likely to change their mind. In our latest CNN/SSRS poll , Biden scores 37% among those potential Democratic primary voters who say they are definitely going to vote for the candidate they currently support. That's well above the 28% he gets overall. Sanders exhibits a similar pattern: He earns 17% overall and 25% among those who say they will definitely vote for the candidate they currently support.

The finding of this particular question is backed up by the consistency of Biden's and Sanders' support. Biden has averaged 30% in 10 CNN polls taken of the 2020 primary. His 28% this time is well within the margin of error of that 30%. Only twice has Biden's support fallen outside the margin of error of 30%: Once, when he received an announcement bump in late April, and later, after his poor performance in his first debate. In both cases, he returned to within 2 points of 30% in the following poll.

Sanders has been even more steady. He's averaged 16% in the 10 CNN national polls. In nine of those 10 polls, he was within 2 points of that 16%. The one exception was when he was at 19% and within 3 points of that 16%.

Now, the current steadiness of Biden and Sanders isn't guaranteed to hold just because it has so far. Indeed, about 40% of voters who support either candidate say they may change their mind.

Read More