Could Trump's tax return response sink him? These voters thought so.

If Monday night's first presidential debate offered a turning point for undecided Pennsylvania voters, it may have come with just two words:

Tax returns.

Up until that topic, Hillary Clinton and Donald Trump seemed to each be holding their own, at least according to a focus group of 27 Pennsylvania voters enlisted and monitored by CBS News at the Constitution Center.

Trump "started off well," said Janice Kearney, 53, a Republican from Audubon. "I think he lost his game in the middle of it."

Others agreed. Each had been armed with dials to record their approval or disapproval as the nominees spoke.

But it was Trump's response to moderator Lester Holt's questions about his refusal to release his tax returns - and Clinton's repeated jabs on the issue - that sent his approval ratings dipping.

It was clear from their reactions, said Frank Luntz, the pollster who ran the focus group, that Trump began to lose the debate when he tried to explain that he was not releasing his tax returns - as presidential candidates have done for decades - because he was still being audited.

And his numbers didn't rebound for the rest of the night.

"His answer to his tax returns failed miserably," Luntz later told reporters. "I don't know if he can recover from this debate."

For almost two hours, the 27 voters sat on risers in a room at the center, flanked by cameras as they watched the debate on a television screen - and others watched them. Eleven participants were women, 24 were white, and three were African American. Each was paid $125 to participate.

Of the group, 11 acknowledged being truly undecided between the two nominees as the night began. The others admitted leaning toward Clinton or Trump.

Immediately after the debate ended, a majority of participants told Luntz Clinton had won. About half still remained undecided, but more were leaning strongly toward Clinton than before the debate began.

Many called Trump's performance poor.

"If he was a student in my middle-school classroom, after I finished telling him to answer the question I asked, I would then say, 'and shut your mouth,'" said teacher Judy McDermott, of Springfield, Delaware County.

But while Clinton was more eloquent and had a more favorable performance in the debate, focus group members said, they did not necessarily all like her answers.

"She speaks well, but I don't think that we really learned too much information," said Jerry Mangold of Philadelphia. "I'm just so angry . . . there was nothing new tonight."

Mangold, who described himself as an independent voter and said he works in sales, was the only participant who said afterward he was very convinced he'll vote for Trump.

Trump's long answers, Mangold said, could be blamed on Holt, the moderator.

Early in the debate, Clinton did better with voters who leaned toward her than Trump did with voters who leaned toward him. But Trump got higher approval with the truly undecided voters than Clinton.

Susan Coleman, a health coach who lives in New Hope, Bucks County, said she wanted to like Trump in the debate. But afterward, she was more convinced to vote for Clinton.

"I was very surprised tonight," she said about liking Clinton more than Trump. Coleman, 54, voted for Mitt Romney in 2012 and President Obama in 2008.

Still, she said, she could still change her mind.

"I really believe Trump has that ability" to be president, she said, "if he can rule out that little whiny guy."

lmccrystal@phillynews.com

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