Sen. Bernie Sanders (I-Vt.), a White House contender in 2016, is known for his stances on budget issues and war. Here are his takes on Obamacare, Social Security and more. (Julie Percha/The Washington Post)

Bernie Sanders's big crowds and polling momentum in Iowa and New Hampshire beg some big questions about the Vermont senator's presidential hopes.

Specifically: What types of Democrats are drawn to him, and will they be enough to help Sanders actually challenge Hillary Clinton.

A new Washington Post-ABC News national poll offers a fresh look at Clinton's and Sanders's standing among Democrats. The survey finds Clinton is overwhelmingly popular across the Democratic Party, but Sanders is a far-less-familiar pol and is weak among a handful of key voting blocs.

Overall, 82 percent of Democrats have a favorable view of Clinton, while 15 percent are unfavorable (a scant 3 percent have no opinion). Sanders's favorable rating is 36 percent among Democrats, with even more offering no opinion of him. Nearly a quarter -- 23 percent -- give Sanders negative marks. That's notable because, despite being less well-known than Clinton, his negatives are eight percentage points higher than Clinton.

Moderates and conservatives
Liberal Democrats have fueled Sanders's rise in early states. And in this poll, liberals are also most the positive about him -- a plus-29 net favorability rating (percent favorable minus percent unfavorable). But Clinton shows very little weakness among liberal Democrats, with a net favorability rating of plus-73 that actually outpaces her standing among moderate and conservative Democrats (plus-62).

Moderate and conservative Democrats mark the first trouble spot for Sanders. Among those voters, equal numbers give the self-described socialist negative and positive ratings (25 percent apiece).

Non-college graduates

A second group where Sanders is weaker are Democrats without college degrees. Forty-six percent have no opinion of him, and the rest split about evenly with a plus-4 net favorable rating. Sanders is far better liked among college graduates, with a net plus-36 favorability rating.

Clinton is far more popular among both groups -- plus-69 among non-college Democrats and plus-62 among college grads.

Non-whites

Sanders's third soft constituency is non-white Democrats. His favorability is positive among white Democrats at plus-19, but among non-whites he's a narrow plus-5. Clinton garners a plus-77 net favorability rating among non-whites and plus-57 among whites. That's a difference for sure, but she's overwhelmingly popular among both groups.

Sanders's lagging popularity might shift as Democrats become more focused on the primaries, but the early signs point to a challenge connecting with Democrats beyond the liberal base.

Given many Democrats simply don't know Sanders, the difference in views of him by subgroup are not gigantic. But lagging support from these groups could prove troublesome. Moderate and conservatives accounted for an average 53 percent of Democratic primary voters in 2008 across all states with exit polls, and non-college graduates accounted for 54 percent of the electorate. Non-whites made up a smaller share -- 35 percent -- though they proved critical in Obama's victory.

Perhaps even more important is Clinton's persistent popularity across all swaths of the Democratic Party. It's far from new, but it shows fellow partisans have not soured on her despite a string of revelations about her personal e-mail and donations from foreign governments to the Bill, Hillary and Chelsea Clinton foundation while she was secretary of state.

Among Democrats so far, Clinton is more than likable enough.

Peyton M. Craighill contributed to this report.