VIRGINIA BEACH — In the Trump era, the suburbs have been Democrats’ surprising superpower.

A revolt by college-educated voters, largely women, in suburbs from Virginia Beach to Oklahoma City, from Houston to Southern California, delivered the House majority to Democrats in 2018. Driven by anxiety over guns, health care and the environment, and recoiling from President Trump’s caustic leadership, suburban voters are widely seen as a critical bloc for any Democratic victory in 2020.

But there are some early signs that the rise of Senator Bernie Sanders, by far the most liberal Democratic front-runner since George McGovern in 1972, is causing stress with the party’s suburban coalition and especially its core of college-educated white women and older voters, many of whom are politically moderate.

And after Saturday night’s big win by Joseph R. Biden Jr. in South Carolina, Mr. Sanders will face an invigorated former vice president as well as other moderates, like former Mayor Michael R. Bloomberg, in Tuesday’s primaries in Virginia, Texas, and additional states with swaths of suburban voters.

Anne Poague, a retired nurse who lives in Virginia Beach, in a House district that a Democrat wrestled from a Republican incumbent in 2018, noted that Mr. Trump’s top argument for re-election would be the economy — with Mr. Sanders as a perfect foil.