Casey's lead was about half of his margin in an August Inquirer poll, and he has fallen below the 50 percent support level needed to win.
Jeffrey Plaut, leader of Global Strategy Group, the Democratic half of the Inquirer polling team, said that though the race had "definitely tightened," Casey still appeared "in good shape to win."
Adam Geller of National Research Inc., the Republican half, saw Casey's support as soft and said history showed that a majority of undecided voters - 14 percent, as of now - were apt to tilt to the challenger in the end. He said that Smith still had a chance and that "the ball can take funny bounces."
In the race for state attorney general, the poll showed Democrat Kathleen Kane leading Republican David Freed by 12 points, 41-29.
Despite Kane's campaign criticism of Gov. Corbett for his handling of the Jerry Sandusky child-abuse investigation when he was attorney general, the governor's popularity has improved since August, although it is still in negative territory.
In terms of voter interest, the Senate race is a far cry from 2006, when Casey ousted two-term incumbent Republican Rick Santorum in a loud, divisive contest that was followed coast to coast.
This time, Casey is up against a political newcomer who has mainly made himself known by spending millions of dollars on TV advertising. Most of the Smith cash comes from the wealth he accumulated as the owner of coal mines in his native Western Pennsylvania.
After 10 years in office - eight as auditor general, two as state treasurer, and now six as senator - Casey appears generally well-liked but still vaguely defined in the minds of many voters.
Only 17 percent of voters say they have a strongly negative opinion of Casey. On the other hand, only 14 percent say they have a highly favorable opinion of him.
The Smith campaign has cast Casey as a spend-happy, do-nothing "Senator Zero" in TV ads.
Poll respondent Dana Venugopalan, a home-schooling mother from Westtown, Chester County, said she'd seen enough to persuade her to vote for Smith. "I'm looking at the broader platform of the Democrats, and I'm saying that's not the direction I want to go," she said. "I'm not looking for more programs to go in government hands."
Said pollster Plaut: "Casey has taken a lot of incoming fire in a short period of time. Television advertising can, and does, take a toll. Casey is still in a strong position."
Until Oct. 1, Casey had withheld ad dollars in the Philadelphia media market, hoarding his cash for the stretch drive.
He cites a record of Senate success. He says he "led the fight" to pass a payroll tax cut, led the effort to add $250 million for support of pregnant women to the health-care law, and, on the home front, led the effort to gain federal funding to dredge the shipping channel in the Port of Philadelphia.
In his ads, he has cast Smith as a "tea partier" too far right for Pennsylvania's generally moderate voters.
"The tea party of Smith just scares me," said retiree Annis Townsend, a poll respondent from Wallingford in Delaware County. "He just sounds way too right for me. I guess I'm influenced by TV commercials."
Casey and Smith have done equally well in consolidating their party's voters behind them. Among Democrats, 72 percent say they'll vote for Casey; 14 percent for Smith. Casey's top strength is in the five-county Philadelphia region.
Among Republicans, 71 percent say they'll back Smith; 16 percent, Casey. Smith is from rural Armstrong County, northeast of Pittsburgh, a region where he hopes to pick up conservative Democratic support.
Democrats have about 4.2 million registered voters in Pennsylvania; Republicans, about 3.1 million.
Independent voters appear to be leaning toward Casey, the poll found. It also found that Casey, like fellow Democrats Kane and President Obama, holds a strong lead among women.
Smith has moved into a statistical tie with Casey among male voters. Geller called that "a scary number if you're Bob Casey" and said it showed that Smith had room to grow.
In the attorney general's race, higher percentages of poll respondents had no strong opinion of either Kane or Freed.
Kane's lead was largely unchanged since August, when she was up by 11. She leads among women, 44 percent to 26 percent, but also among men: 39 percent to 32 percent. The results suggest GOP attack ads painting her as a "weak" rape prosecutor (a claim debunked by fact-checkers) may have had little impact.
The poll found that 42 percent of voters approved of the job Corbett is doing as governor, while 45 percent disapproved.
That's an improvement for Corbett, who in recent months has shuffled his staff and sought to be more accessible to voters.
In August, 52 percent disapproved of his work, and 38 percent approved.
The Inquirer this week also surveyed the Senate race in New Jersey. Democratic incumbent Robert Menendez held a 14-point lead over GOP challenger Joseph Kyrillos.
Contact Tom Infield at 610-313-8205, firstname.lastname@example.org, or follow on Twitter @tinfield.