The Big Tent Project, a Democratic political group focused on promoting moderate presidential candidates, has sent hundreds of thousands of mailers bashing Bernie Sanders to black voters in South Carolina who voted in the state's 2016 primary.

Why it matters: Sanders' rise to the top of the pack, as dueling moderate candidates split their side of the vote, is worrying many in the Democratic political establishment who fear a socialist can't beat President Trump.

Sanders' triumph in Nevada, after rising to the lead in Iowa and New Hampshire, creates the very real potential that Sanders will be the Democratic nominee.

What's next: His performance in South Carolina will test whether his appeal with young voters also translates to black voters — and deprives Joe Biden of votes critical to his campaign’s survival.

Courtesy of The Big Tent Project

The details: The mailers, which will go to 215,000 people and cost roughly $100,000, charge that several of Sanders' most ambitious policy proposals — such as free college tuition — are unrealistic and wildly expensive.

They also assert that "Trump will crush Bernie on taxes and spending" and conclude: "Nominating Bernie means we re-elect Trump."

The Sanders campaign did not respond to requests for a comment.

The big picture: This mailer is one piece of the efforts by Big Tent — a 501(c)(4) — (C4s are able to mask their donors).