(CNN) One of the arguments Bernie Sanders' fans made during the 2016 Democratic primary was that he was more electable than Hillary Clinton. His favorable ratings with the general electorate were far higher than Hillary Clinton's. Indeed, Sanders maintained fairly high favorable ratings with all voters as late as 2018.

Sanders' popularity among all voters seems to be declining considerably in the last few months, however.

Our new CNN poll puts Sanders favorable rating at 46% compared to an unfavorable rating of 45% among registered voters. This is only the latest poll to have Sanders at basically even in his net favorability rating (favorable-unfavorable). A Quinnipiac University poll from late December gave the Vermont senator a net favorability of just +2 points. An average of all recent polls put Sanders' net favorability at about -1 points.

Compare that to where Sanders was at the end of his 2016 presidential bid. Sanders had a 59% favorable rating to 36% unfavorable rating among all voters in a CNN poll taken in June 2016

Sanders was able to hold onto much of his popularity through last year. A CNN poll taken in early December 2018 gave him a +13 net favorability rating with all voters. A Gallup poll in September 2018 had him at a +15 net favorability rating with all adults.

So, what's changed? I'd argue that Sanders was benefiting from not being in a competitive campaign. (Former Vice President Joe Biden, who has garnered the most support in general election polls among the Democrats, may be benefiting from a similar effect.) When you're not being thought of a viable threat to win a party's nomination, opponents tend to lay off. The last time Sanders was thought of as at least a minor threat to win the Democratic nomination was in March 2016. His net favorability rating back then among all voters was +3 points in a CNN poll

The good news for Sanders is that his net favorability rating is at about the same level as the other people who have declared their candidacy for the Democratic nomination. Most of these other candidates, however, are relatively unknown. At least at this point, it's not the case that Sanders is less electable than the average Democrat. It's just that he cannot make the case that he is more electable based on national polling.

Sanders, though, may have to convince Democratic voters that he electable. In our poll, just 30% of Democratic voters believe the party has a better chance of winning the presidency with him than someone else as the nominee. The vast majority, 59%, think they have a better shot of winning with someone else.

That may change, though, when Biden gets into the race. The big unknown is whether Biden, whose ratings are currently sky high, is also going to come crashing down to Earth if and when he declares. It's probably not a bad bet given past history and what has happened to Sanders. Then Democrats will have a much tougher time figuring out who is the most electable.