Asked if he could see Mrs. Clinton losing any of the March 1 contests in Southern and border states, Mr. Mook did not answer directly, saying, “I’m really excited about the support that we’re holding right now.”

But he emphasized that the campaign was only starting to build a political fortress in the region, with work ramping up in Virginia and Georgia. He added that the campaign was reaching out to voters across all demographic groups.

“Just because you win among African Americans and Hispanic Americans doesn’t mean you can write off white Americans, and vice versa,” Mr. Mook said.

Candidates win delegates based on vote totals, so Mr. Sanders and other Clinton rivals can come away from Southern states with delegates even if Mrs. Clinton wins there. But Clinton delegate counters believe she will be so dominant that she will take the lion’s share of delegates, unlike in 2008, when Mr. Obama often earned sizable numbers of delegates with strong wins and close second-place finishes. The South is the most visible of several firewalls that Mrs. Clinton and her advisers are trying to construct. They are lining up superdelegate endorsements at a brisk pace, with Mrs. Clinton holding a rally on Saturday in the critical state of New Hampshire to highlight the backing of Senator Jeanne Shaheen.

Mr. Mook said that “we’re trying to get every superdelegate,” but declined to say how many of the 713 had committed to Mrs. Clinton. Other Clinton advisers said a sizable fraction had done so — a crucial step toward outpacing Mr. Sanders and Mr. O’Malley in total delegate support, Democrats noted. (A spokeswoman for the Democratic National Committee notes, however, that superdelegates are free to change their minds and support another candidate at any time.)

“Clearly the campaign has learned well from its 2008 experience, and there seems to be a lot more outreach these days both at superdelegate level and at ground level,” said Julián Castro, a former mayor of San Antonio who is Mr. Obama’s secretary of housing and urban development.

The outreach has been aggressive. Jon M. Ausman, a longtime Florida superdelegate, said he had received a call from Craig Smith, an old friend who was political director in the Clinton White House and is close to both Clintons. Blanca O’Leary, a Colorado superdelegate, said she had received calls from “top Clinton people” before pledging her support to Mrs. Clinton. A North Carolina superdelegate, Pat Cotham, said she had recently received phone calls and emails from Clinton aides — including one whose title was the “southern region political states director” and who sent her a pledge form.