Nikki Haley on Saturday blasted Donald J. Trump’s dishonesty and his relationships with “dictators,” questioned his mental acuity and dismissed his mounting stack of endorsements, sharpening her attacks on him as she headed into the final two days of New Hampshire campaigning.

Delivering her most forceful case yet for the Republican presidential nomination, Ms. Haley, the former governor of South Carolina, embraced her underdog status this weekend as independent, anti-Trump voters urged her on.

But with the first-in-the-nation primary on Tuesday, Ms. Haley has enormous ground to make up and very little time to do it. Mr. Trump was filling arenas and event centers in Concord and Manchester, N.H., on Friday and Saturday, speaking to adoring throngs as Republican elected officials fell in line. His event Saturday night in Manchester drew a few thousand fans. Ms. Haley, meanwhile, was visiting retail stores and restaurants. Her largest event, in Nashua, N.H., drew around 500.

Suffolk University’s daily tracking poll of New Hampshire voters on Saturday had Mr. Trump leading Ms. Haley by double digits, 53 percent to 36 percent, with his margin having crept up a percentage point each of the previous two days.

“When you’re dealing with the pressures of a presidency, we can’t have someone else that we question whether they’re mentally fit to do this — we can’t,” Ms. Haley said in Keene, N.H., criticizing Mr. Trump for confusing her with Nancy Pelosi at his Friday night rally. “So, that’s the choice. Do you want to win in November or not? Do you want to be scared in November or not? Do you want your kids to be proud in November or not?”

Later, in a news conference with reporters in Peterborough, N.H., Ms. Haley questioned whether Mr. Trump would be “on it” enough to lead the nation. “My parents are up in age, and I love them dearly,” she told reporters. “But when you see them hit a certain age, there is a decline. That’s a fact — ask any doctor, there is a decline.”