Senate Majority Leader Dominic Pileggi surprised a number of party leaders with his announcement last week that he would consider a bid, but he quickly ended speculation with a statement on his Facebook page that thanked people who encouraged him to run. He did not say why he decided against it.
Pileggi probably would have become the highest-profile Republican to run. However, he had no guarantee of winning the state party endorsement, he likely would have faced a conservative challenge in the April 24 primary and he would have had to give up his state Senate seat, or face answering why he's running for two offices at once.
"After a great deal of careful consideration, I have decided not to enter the race for the United States Senate," Pileggi wrote on Facebook. "I will continue to focus on my service as Senate majority leader, working for positive change in state government."
Pileggi, of Delaware County, is widely viewed as a moderate from a part of the state that is rich with established party donors - but one opponent had suggested that that wouldn't have helped help him in a primary traditionally dominated by conservative voters.
So far, at least 10 others have said they will seek the Republican nomination to take on Casey.
They are entrepreneurs Tim Burns and Steve Welch; manufacturing executive David Christian; tea party activist Laureen Cummings; pharmacist John Kensinger; former state Rep. Sam Rohrer; lawyer Marc Scaringi; former coal industry executive Tom Smith; retired U.S. Army Col. John Vernon; and retired US Army Sgt. Robert Allen Mansfield.
At least two, Smith, who has considerable personal wealth to tap into, and Rohrer, who lost to Gov. Tom Corbett in last year's Republican primary, plan to stay in the race through the primary, with or without the party endorsement, aides say.
However, none has statewide name recognition or the ability to claim the GOP's undivided backing, and only Rohrer has a substantial background in elected office or experience as a candidate in a statewide election.
Dec. 31 is the next deadline for federal candidates to report campaign finances, and party leaders will be watching to see who can raise enough money to rival Casey, and who can't.