Bernie Sanders

WASHINGTON — Vermont independent Sen. Bernie Sanders is preparing an aggressive push to regain ground in the race for the Democratic presidential nomination, combining a series of ads, policy speeches and appearances before key groups of voters in a bid to ignite his challenge to front-runner Hillary Rodham Clinton.

Much of his effort will focus on attracting minority supporters, starting this weekend with events with black voters in South Carolina and Hispanics in Nevada.

A wild card remains in how Sanders chooses to address Clinton’s use of a private e-mail system while serving as secretary of state. After seeming to set aside the issue of what he described as her “damn e-mails” in the first Democratic debate, he appeared to reopen the saga in an interview with The Wall Street Journal, saying there are “valid questions” about her correspondence.

That prompted a sharp response from the Clinton campaign, with spokesman Josh Schwerin accusing Sanders and his strategists of engaging “in the type of personal attacks that they previously said he wouldn’t do.”

Tad Devine, Sanders’ chief political strategist, pushed back against the notion that Sanders had shifted his approach.

“Bernie’s never gone there and I don’t see us going there in the campaign unless it gets incredibly nasty and they make outrageous accusations about him,” Devine said. “I hope that doesn’t happen.”

The exchange underscores the central challenge for Sanders as he attempts to boost his support ahead of the Iowa caucuses — how to draw distinctions with Clinton without violating his pledge to avoid character attacks.

His speech last month in Iowa during the party dinner included a series of sharp contrasts with Clinton on issues important to the Democratic base, such as the Keystone XL pipeline, the Trans-Pacific Partnership trade deal, gay marriage and the Iraq war.