PHILADELPHIA — More than two dozen undecided Pennsylvania voters began watching the first debate between Hillary Clinton and Donald Trump Monday feeling deeply dissatisfied with their choice, describing Trump as “scary” and an “egomaniac” and Clinton as “corrupt” and “disingenuous.”

After 90 minutes, the participants were not necessarily won over by the candidates. But they overwhelmingly believed Clinton had seized the moment, coming off as “firm” and “knowledgeable,” the more presidential of the two. Trump, they said, was “defensive” and “bombastic,” leaving “unanswered questions” while dilly-dallying on difficult topics.

“He went off message,” said one man.

A woman echoed, “I was really hoping to see a different Donald Trump tonight.”

Asked whether the outcome was more reflective of Clinton’s success or Trump’s failure, the participants agreed Trump had fallen short. “He took the bait,” said one woman, while others suggested he was ill prepared for the matchup.

The focus group, moderated by the pollster Frank Luntz, comprised 27 undecided voters from one of the key battleground states in this election, which recent polling has shown to be deadlocked between Clinton and Trump. (The RealClearPolitics average shows Clinton ahead by 1.8 percentage points.)

The performance Monday left five participants strongly favoring Clinton who initially had not. Still, most of the group did not think this debate would mark a fatal blow to Trump’s candidacy, and most planned to watch future debates before making a final judgment.

But enthusiasm is lagging for both nominees. Prior to the debate, the focus group participants bemoaned the choice before them, reflecting the high unfavorable ratings that have dogged Clinton and Trump nationally. “It’s like asking me to choose between a heart attack and a stroke,” said one woman.

One man said he has supported Republican candidates in every election since he was 18 years old, but expressed deep reservations about Trump. “If any Republican had been nominated other than Trump,” he said, “there’d be no question.”

The debate, moderated by NBC News anchor Lester Holt at Hofstra University on Long Island, was seen as a prime opportunity for Trump to assuage some of those concerns among people inclined to support him or uncomfortable with Clinton. Most of the focus group’s participants said beforehand they were most interested in what the Republican nominee would have to say.

In his more measured moments, Trump seemed poised to meet their expectations, receiving high marks on responses like those regarding the Iran nuclear deal and trade. But their impressions tanked when Trump became defensive or attacked Clinton personally.

When Trump said he has “a much better temperament than she has," for example, he earned the lowest overall score of the night among the focus group, with all participants rating it very negatively.

The discussion of Trump’s tax returns, which he has refused to make public, also did not rate well with the group. When Trump shifted the discussion to Clinton’s emails, the response suddenly spiked positive — but Clinton successfully moved the discussion back to Trump’s tax returns, and the tide turned back.

“Donald Trump just got nuked,” Luntz observed to a group of reporters, “and I don’t know if he can recover from this debate.”

Still, the performance was not an unqualified victory for Clinton, at least among this select group of voters in Philadelphia. Although many participants agreed that her responses were practiced and measured, one man noted, “I don’t believe her.” Six people in the group said after the debate that they would likely not vote at all.

Their decisions could be highly consequential because they reside in Pennsylvania, a swing state that backed President Obama in 2008 and 2012. Underlying the tight race has been a pronounced shift by white, working-class voters toward Trump, a trend echoing in states such as Ohio and Iowa.

But the race here and in other battlegrounds remains unusually fluid because a historic proportion of would-be voters are profoundly dissatisfied with both candidates. The dynamic would seem to imbue the three presidential debates, and one vice presidential debate, with even more importance.

Before the debate Monday, the focus group participants laid out what they hoped to see from each candidate. Clinton, they hoped, would “show us she’s being real and honest” and focus more on policy than on Trump. They were optimistic Trump would “be presidential.”

This time, the focus group suggested Clinton came closer to her objective. The candidates will now have two more debates to cement or shift these impressions.