Word of the latest poll numbers has already produced an influx of cash donations from around the country, Jim Conroy, Smith's campaign manager, said Friday.
"Our online fund-raising has gone through the roof," he said.
Casey, a former state auditor general and former state treasurer with four statewide victories under his belt, has been outspent by the political newcomer.
The Casey camp, monitoring its opponent's ad buys, says Smith has spent $4.2 million on commercials since the April Republican primary, in which he outspent several GOP rivals.
Casey, with "limited resources," has been hoarding much of his campaign cash for the final weeks of the contest and has spent $1.5 million on TV to date, his campaign manager, Larry Smar, said.
Casey started running TV ads in Philadelphia only on Friday. Until then, Smar said, the candidate hadn't spent a nickel in the state's largest TV market. He now hopes to blunt any further Smith gains.
"Smith has been saying for a long time that this is going to be a $20 million race," Smar said. "He has made clear that he is going to keep dumping money into it."
Smar said Casey had been keeping up with Smith in the Pittsburgh TV market, where ad rates are less expensive.
The last time either of the candidates officially issued a public report on campaign finances was in July. The next report isn't due at the Federal Election Commission until Oct. 15.
A Muhlenberg College/Allentown Morning Call poll made public Friday showed the effects of Smith's ad blitz.
The relatively small survey of 427 likely voters, with an error margin of plus or minus 5 percentage points, reported Casey with 44 percent support and Smith with 36 percent backing. The remainder of voters - 20 percent - were undecided or didn't like either candidate.
Casey's 8-point lead in the Muhlenberg poll compared with a 12-point lead earlier in the month and a 19-point lead in August.
A Quinnipiac University poll of likely voters released Wednesday had Casey up by six points. A Franklin and Marshall College poll the same day put the gap at 10.
Christopher Borick, the Muhlenberg poll director, said the race clearly was narrowing.
"We have a situation in which an incumbent senator with a golden name has seen his standing slip poll by poll," Borick said. "I think it's reasonable to start wondering what is going on."
Casey, low-key by temperament, has run a low-key race.
"Campaigns take work, campaigns take effort, and up to now, you'd have to say that the public effort - the visible effort - from the Casey folks is not very obvious," Borick said.
Smith on Friday got a chance to appear with Mitt Romney at a fund-raising event in Philadelphia. He later made an appearance with the Republican presidential nominee before hundreds of people at Valley Forge Military Academy and College.
In an interview Tuesday, Smith declined to say how much he ultimately might spend on the race.
"So far, so good," he said. "Sen. Bob Casey is not that strong. No matter what poll you look at, he cannot get over 50 percent, and for a sitting United States senator, that is pretty weak."
Casey, in an interview last week, said Pennsylvania political history had taught him to expect the race would tighten.
He beat incumbent Republican Rick Santorum by more than 17 points, 58.7 to 41.3 percent, in 2006. But Senate races in the state are often in the "52 to 48 range," he said.
Casey's aides said the new polling data simply may show some Republican voters learning about Smith and declaring they'll vote for their nominee.
Democrats outnumber Republicans in the state, 4.2 million to 3.1 million.
"It's still pretty early and we've got a long way to go before Election Day," Casey said. "I have seen races in our state that have changed very quickly and you've got to be ready for change."
Contact Tom Infield at 610-313-8205, firstname.lastname@example.org, or follow on Twitter @tinfield.