Advisers to Hillary Clinton, including former President Bill Clinton, believe that her campaign made serious miscalculations by forgoing early attacks on Senator Bernie Sanders of Vermont and failing to undercut his archliberal message before it grew into a political movement that has now put him within striking distance of beating Mrs. Clinton in Iowa and New Hampshire.

According to Democrats close to the Clintons and involved with her campaign, Mrs. Clinton and the former president are also unnerved by the possibility that Mr. Sanders will foment a large wave of first-time voters and liberals that will derail her in Iowa, not unlike Barack Obama’s success in 2008, which consigned Mrs. Clinton to a third-place finish. They have asked her advisers about the strength of the campaign’s data modeling and turnout assumptions in Iowa, given that her 2008 campaign’s predictions were so inaccurate…

But Mrs. Clinton’s problems are broader than just her message: Opinion polls show that some Democrats and other voters continue to question her trustworthiness and whether she cares about their problems. Recent polls show that her once-formidable lead over Mr. Sanders in Iowa has all but vanished, while he is holding on to a slight lead over her in New Hampshire.

Mrs. Clinton and her team say they always anticipated the race would tighten, yet they were not prepared for Mr. Sanders to become so popular with young people and independents, especially women, whom Mrs. Clinton views as a key part of her base. Given her many political advantages, like rich donors and widespread support from Democratic Party elites, she is also surprised that Mr. Sanders’s fund-raising has rivaled hers and that her experience — along with her potential to make history as the first woman elected president — has not galvanized more voters.