By Edith Honan
NEW YORK (Reuters) - The U.S. Senate race in Pennsylvania has become "too close to call" three weeks before the election, while President Barack Obama's lead in the state has slipped from 12 to 4 percentage points after a disappointing debate performance, a Quinnipiac poll said on Tuesday.
Pennsylvania, seen until recently as secure for the Democratic president, now appears to be competitive, with Obama leading Republican Mitt Romney 50 percent to 46 percent. Just 7 percent of the state's likely voters say they might change their minds before November 6, the poll found.
The large state is one of the top prizes in the presidential election with 20 votes in the Electoral College, tying it with Illinois for fifth place in the number of electors by state. Only California, Texas, New York and Florida have more.
"It's no secret that the president underperformed in the debate, including by his own account," Tim Malloy, assistant director of the Quinnipiac University Polling Institute, said in an interview.
While Pennsylvania-born Vice President Joe Biden's debate performance last week against Romney's running mate, U.S. Representative Paul Ryan, played well with voters in the state, it "did not move the ticket at all," Malloy said.
In Pennsylvania, men, white Catholics and blue collar voters have flocked to the Romney camp over the last month, he said.
In Quinnipiac's September 26 poll, Obama led by 12 points.
Obama and Romney will face off in their second of three debates later on Tuesday.
In the U.S. Senate race, the poll spells bad news for Democrats, who hope to hold on to their 51 to 47 advantage over Republicans in the 100-seat chamber. While much attention has been paid to a handful of close races - including Missouri, Indiana, Massachusetts and Connecticut - Democratic incumbent Robert Casey Jr. has been favored to win in Pennsylvania.
Casey, an anti-abortion, pro-gun moderate Democrat whose father was a popular Pennsylvania governor, won the Senate seat six years ago by defeating Republican Rick Santorum by 18 percentage points. Santorum went on to be one of Romney's principal rivals for the Republican presidential nomination.
Republican businessman Tom Smith was virtually unknown in the state in April when he won the Republican Senate primary. As recently as August 1, a Quinnipiac survey showed Casey beating Smith 55 percent to 37 percent.
Smith, a former coal miner who went on to own a coal company, has poured nearly $17 million of his own money into his campaign and has spent heavily on television ads. According to the poll released on Tuesday, Casey's lead has narrowed to just 3 percentage points.
"Tom Smith's relentless TV ad barrage has lifted him out of the coal mine to give Sen. Robert Casey a run for his money," said Malloy.
According to campaign finance filings on Monday, Smith outraised Casey by more than $100,000 in the last quarter, not including $10 million he loaned his campaign in the same period.
The telephone survey of 1,519 Pennsylvania likely voters was conducted from Friday to Sunday and had a margin of error of plus or minus 2.5 percentage points.
(Editing by Paul Thomasch and Doina Chiacu)