The battle between Hillary Clinton and Bernie Sanders is heating up. There looks to be a great deal of enthusiasm for Sanders on the ground in Iowa, and both early state and national polls are getting quite close. This has led Clinton to lash out with negative attacks against Sanders.

Nationally, Clinton is still clearly in front among Democrats and Democrat leaners, according to this week's NBC News/SurveyMonkey Weekly Election Tracking Poll, which was conducted online — she leads 53 to 36 percent. But the crosstabs of the poll reveal three main demographic fault lines between Clinton and Sanders supporters: gender, age, and race.

1) Race: Sanders supporters are more white

Interestingly enough, Sanders is actually winning white Democrats and Democratic-leaning voters in this poll — 46 percent of them pick him, and 44 percent pick Clinton.

Yet Clinton remains well ahead due to her overwhelming support among nonwhite voters. Sixty-three percent of black Democrats and Democratic-leaning voters say they support Clinton, and just 20 percent support Sanders. The numbers among Hispanics are 61 percent for Clinton and 27 percent for Sanders. And people who said they were of another race supported Clinton over Sanders 59 percent to 30 percent.

This points to what may be the biggest question about Sanders's prospects in the primaries. If he manages to win Iowa and New Hampshire, can he use the ensuing media coverage of him to hugely improve his margins among nonwhite voters? Many in the Democratic establishment are assuming he can't. But if he manages to do so, Clinton should be worried.

2) Gender: Sanders supporters are more male

For the time being, both male and female Democrats and Democrat leaners are more likely to support Clinton than Sanders. But the margins are far closer among men, where Clinton leads by just 6 percentage points. Among women, however, she racks up a 56-to-32-point margin.

This gender gap has been evident in the polling for a while. Some attribute it to sexism among men, some to women naturally being more likely to support the person who would be the first woman president. Whatever your explanation, it's been persistent in polls for months now.

3) Age: Sanders dominates among the youngest voters but trails among older ones

For the most part, the older Democrats get, the more likely they are to be skeptical of Bernie Sanders. The Vermont senator dominates among Democrats and Democratic-leaning 18- to 24-year-olds, winning 65 percent of them compared with Clinton's 27 percent.

But 25- to 34-year-olds are evenly split among the two, and every age category older than them supports Clinton by overwhelming margins. (Among Democrats older than 65 years old, Clinton leads by 25 points.) Perhaps older Democrats who remember the Clinton administration are more likely to have affection for Hillary Clinton — or perhaps they're more deeply skeptical about whether a "democratic socialist" can win a general election.

Correction

A previous version of the article and charts stated that black voters support for Clinton and Sanders splits 73% and 13%. The correct numbers 63% and 20% have been updated.