MANCHESTER, N.H. — Hillary Clinton has been here before, watching a political rival generate youthful enthusiasm with lofty proposals she believes are going unchallenged.

So on Thursday, she took matters into her own hands.

In the first head-to-head debate between Clinton and Bernie Sanders, the former secretary of state tried to systematically undermine the Vermont senator’s plans for a government-run health care system, his call for free college tuition and his foreign policy judgment. And she upbraided Sanders for his “artful smear” of the high-dollar speaking fees she received from Wall Street banks and the implication she’s beholden to the financial firms as a result.

“I really don’t think these kinds of attacks by insinuation are worthy of you,” Clinton said to Sanders. “If you have something to say, say it directly.”

Clinton’s fiery performance was a manifestation of the frustration growing inside her campaign for weeks. Her team believes Sanders is getting away with breaking his pledge to avoid negative attacks. And they think he’s not being straight with Americans about the cost of his proposals, particularly his call for a single-payer health care system.

With the attention of a prime-time television audience, Clinton set about trying to vet Sanders’ proposals on her own. When it comes to his calls for broader government involvement in domestic programs, Clinton said, “The numbers don’t add up.” On foreign policy, she tried to poke a hole in Sanders’ argument that his vote against the invasion of Iraq is proof he has the judgment necessary to manage America’s involvement in the turbulent Middle East.

“A vote in 2002 is not a plan to defeat ISIS,” she said.

Addressing her own readiness to be president, Clinton said: “I’ve been vetted. There’s hardly anything you don’t know about me.”

While the contest between Clinton and Sanders has grown more heated in recent weeks, Thursday’s debate took the rancor to a new level. It followed Clinton’s narrow victory in the Iowa caucuses and came just days before the primary in New Hampshire, where Sanders has for months maintained a large lead in preference polls.

Clinton’s allies don’t believe her standing is nearly as precarious as it was during this stage of her 2008 primary fight against Barack Obama. They view Sanders as a candidate with narrower appeal, particularly once the primary calendar moves to states with more racially diverse electorates.

But there are enough eerie echoes of that contest eight years ago to spark growing anxiety, as well as a determination to not underestimate Sanders.