(CNN) First things first: The theme song of the week is the theme from The Mary Tyler Moore Show .

It shouldn't be too surprising that Sanders' decline is associated with an increase in Warren's numbers. Sanders and Warren are the two most progressive major candidates in the Democratic field . Both have consistently polled their best among those Democrats who call themselves "very liberal" and their worst among Democrats who call themselves either moderate or conservative.

By merely looking at ideology, however, you miss what I believe are key differences between the types of voters each is attracting. It could prove difficult for Warren to make further gains among Sanders' supporters, unless she starts appealing to a different type of voter.

Sanders' voters are more likely to be mirror images of President Donald Trump's voters: working class and fed up with the current dysfunction in Washington, just on the other end of the political spectrum

Warren's voters are certainly liberal, but they are well-educated and aren't anywhere near as anti-establishment.

Take a look at the latest Fox News poll. Among white voters with a college degree, Sanders has 13% support. His support rises to 23% among white voters without a college degree.

Warren's support is the reverse: She has 14% among white voters with a college degree and 7% among those without one.

In this way, Sanders is actually a lot more like Biden. Both do better among white voters without a college degree than with those with one. This could partially explain why the plurality of Sanders' voters say their second choice for the nomination is Biden, not Warren. It's also why I wrote last week that Biden was eating Sanders' 2016 base (when he won whites voters without a college degree over Hillary Clinton.)

Some of Warren's difficulties may because white voters without a college degree are more likely to hold sexist views . But it's not just a socioeconomic divide that splits Sanders and Warren voters.

Our last CNN poll asked voters whether they approve or disapprove how Democratic leaders in Congress are handling their jobs. By a 52% to 42% margin, Democratic primary voters said that they approved.

Sanders is not pulling the bulk of his support from that majority. He gets 11% from those who approve -- and 18% among those Democrats who disapprove of the job the congressional Democratic leadership is doing.

It's the opposite for Warren. She earned 9% among those Democratic voters who approve of the job that the Democratic leadership in Congress is doing. She pulls in just 5% from those who disapprove.

As was the case in 2016, Sanders' backers are not pleased with business as usual. He gets 22% from those who think it at least very important to bring an outsider's perspective to Washington, despite being in Congress for nearly 30 years. Warren, in Congress since 2013, is only at 6% among this group. Among those who do believe it is only somewhat important or not important, Sanders takes in just 11%. Warren stands 8% with them.

Sanders' profile actually matches Biden more closely than it does with Warren. Despite Biden serving in the federal government for more than 40 years, Biden wins 45% from those who say it is at least very important to bring an outsider's perspective to government compared to 33% who believe it is only somewhat important or not important at all. This could, of course, be a statistical quirk.

The question could be interpreted as a stand-in for the belief that government isn't working as it should. Biden's bipartisan pitch and Sanders' postpartisan appeal both aim at solving that problem. Warren, on the other hand, is viewed as more of a harsh partisan . (See her refusal to have a Fox News town hall .)

Put another way, Warren's task over the next few months is to get at Sanders' base of fed-up working class voters. Right now, Biden's actually the one doing that, and it's part of the reason he's ahead.