Why voters aren't considering candidates and how the debates might change that

In the crowded contest for the Democratic presidential nomination, most candidates struggle to stand out from the field. According to the latest CBS News polling of early primary and caucus states, the top three candidates — Elizabeth Warren, Joe Biden, and Bernie Sanders — are each being considering by at least about half of Democratic likely voters. But for all the other candidates, fewer than half of voters are considering them. 

Why is this so? Here's what the voters said.

After voters told us which candidates they are considering, we randomly presented one of the candidates not under consideration and asked them to choose from a list of reasons why they weren't considering the candidate (voters could pick more than one).

The main reason voters gave for not considering a candidate was that they just hadn't heard enough about them yet. Voters say this half the time.

The next most common reasons given were that the candidate "can't defeat Donald Trump" (31%) -- perhaps unsurprising, given that Democrats have been prioritizing electability -- and that they are "not prepared to be president yet" (26%).

Less common reasons were disagreeing with the candidate's ideas (15%) and concern that the candidate would take the party in the wrong direction (12%).

Thursday night's presidential debate features 10 candidates and is the first time this campaign that all qualifying candidates will be on the same stage. Here's what the "why not" numbers look like for the seven candidates that fewer than half of voters are now considering.

Again, the reason most often given was that voters just haven't heard enough about the candidate yet, with electability concerns ("can't defeat Donald Trump") often close behind. 

The exceptions here were Kamala Harris and Beto O'Rourke, who voters said they've heard more about. In these cases, voters said they have concerns about electability, and in O'Rourke's case, about whether he's prepared enough to do the job.

One thing that debates do is give candidates a platform to introduce themselves to the voters. Which candidates have more room to grow in terms of gaining some familiarity among voters?

One clue is how many voters give "haven't heard enough" as the sole reason for not considering a candidate. Julián Castro, for example, was only under consideration among 17% of Democratic likely voters, which means 83% are not currently considering him. Four in ten of those not considering him said they haven't heard enough about him and give no other reasons. That means that 33% of voters overall aren't considering Castro purely due to a lack of familiarity.

Here's how this measure breaks down for each candidate featured in tonight's debate. Each horizontal bar sums to the total percentage of voters not currently considering the candidate.

The three candidates who appear to have the most room to grow tonight, purely on the basis of making themselves known to the public, are Castro, Amy Klobuchar, and Andrew Yang (whose campaign has promised some theatrics tonight). Pete Buttigieg and Cory Booker are next, with 24% of voters saying they're not considering them only because they haven't heard enough yet.

Of course, there's no guarantee voters will like you as they get to know you. Nonetheless, many candidates are suffering from a lack of name recognition and haven't broken through the noise. As more voters tune into the primary process, candidates still have opportunities to introduce themselves to the voting public and make their case. Tonight's debate in Houston is a big one.

Jennifer De Pinto contributed to this analysis.