WASHINGTON — Hillary Clinton has questioned Bernie Sanders' electability. She's criticized his plans for health care, foreign policy and Wall Street. And she's tagged him with flip-flopping on gun control.

None of it appears to be sticking, say some Democrats who have raised concerns that a months-long primary campaign could create lasting damage for their party.

While most believe Clinton will still capture the nomination, some say she is failing to respond effectively to Sanders, fueling both his primary rise and strengthening the Republican argument against her. Others say she got too late a start going after Sanders and is still not hitting him enough — eroding her lead in states that should be safe.

“They didn't take him seriously enough because they thought they had a gadfly,” said John Morgan, a Florida attorney and Clinton donor. “The gadfly wasn't a gadfly — he was a lightning bug. And people have been following that lightning bug.”

Though Sanders has dismissed questions about Clinton's use of a private e-mail account and server as secretary of state and how she responded to the deadly 2012 Benghazi attacks, some Democrats say she needs a better response to his critique of her ties to Wall Street, which they argue reinforces months of Republican attacks on her character.

“The better she does, the more those stories seem to recede in the minds of voters,” said Boston-based Democratic strategist Mary Anne Marsh. “Now, Benghazi seems to be back. E-mail seems to be back. A number of other things seem to be back.”

Though the tightening nature of the race, particularly in Iowa, hasn't sparked widespread panic among her supporters quite yet, it's worrying some donors, as well as Bill Clinton and their daughter, Chelsea, who shared her concerns at a meeting with supporters in New York this week, according to people close to the campaign, who spoke on condition.

Donors, strategists and pollsters expressed their concerns in nearly two dozen interviews with The Associated Press.

Clinton has long struggled to connect with liberal activists, who have gained influence as the party has shifted left during President Barack Obama's two terms in office. “Clinton's message has always been more effective for a general election audience than the primary activist base,” said strategist Ben LaBolt.