Sen. Arlen Specter's decision to switch from the Republican to the Democratic party in April 2009 seemed like a good idea at the time.
Specter, the five-term Republican senator from Pennsylvania, had taken a lot of heat for being only one of three Republicans to vote for President Obama's stimulus package. Most polls showed that Specter wouldn't be able to defeat his GOP challenger, Pat Toomey, in the 2010 primaries.
"The prospects for winning a Republican primary [in Pennsylvania] are bleak," Specter acknowledged at the time.
But little did Specter, who has fought many a tough election battle, anticipate that he would be thrown into an even tougher Democratic primary with Rep. Joe Sestak -- and that the decision he thought would salvage his 30-year Senate career would be the end of it.
Specter wasn't the only incumbent facing anti-Washington sentiment, but he was the only one who faced the added scrutiny of having changed parties a year before the elections.
"There have really been two huge ingredients, each reinforcing each other," said Terry Madonna, a political analyst and professor of public affairs at Franklin & Marshall College in Lancaster, Pennsylvania. "We've got a party switch and anti-incumbency [sentiment] and those two have blended together to be a very powerful argument for Joe Sestak."
Tom Ridge, former Pennsylvania governor and the first ever secretary of the Department of Homeland Security, said Specter's decision was fraught with risk but his campaign never thought they were going to have a contested Democratic primary.
"The one thing that Arlen and his team have done over the years is they have run tough, hard-nosed campaigns and I suspect there was a point in time that given the closeness of the race against [Pat] Toomey, they counted the numbers and saw it was impossible to win," Ridge told ABC News. "I would've preferred Arlen stay and fight for what he believed in. ... but he chose to fight with Pennsylvania on the other side of the aisle and it was a risky proposition."
Not having Specter in one of Pennsylvania's two Senate seats will be a big change, political analysts say. The 80-year-old Senator was first elected to the post in 1980 and has become something of a fixture on the political scene.
"Senator Specter has been in and around the Senate for so long that it's hard to imagine the Senate without him," said Nathan Gonzales of the bipartisan Rothenberg Political Report.
Specter started his career in the Air Force Office of Special Investigations. He became assistant district attorney in Philadelphia in 1959, during which he was part of the Warren Commission that investigated the assasination of President John F. Kennedy in 1963. In the 1970's, Specter was elected district attorney of Philadelphia, during which time he tried numerous times -- unsuccessfully -- to run for public office. His luck changed when, in 1980, he beat eight Republican candidates and secured his place on the GOP ticket for Senate. Specter defeated Democrat Peter E. Flaherty, former mayor of Pittsburgh, to take over the seat he would occupy for the next 30 years.
Primary Defeat Ends Arlen Specter's 30-Year Senate Career
The five-term Specter kept his Senate seat warm for decades, but it hasn't been an easy ride all the way.
His 1992 Senate run was nearly derailed after he aggressively questioned Anita Hill during confirmation hearings for Supreme Court Justice nominee Clarence Thomas, earning the ire of Democrats. Most recently, Specter won by less than a 2 percent margin in the 2004 Republican primary against the conservative Toomey; he was helped by the last-minute endorsement of President George W. Bush.
"He's worked his tail off," Ridge said. "He has survived many close calls and one thing Arlen has been in 30 plus years is untiring."
Specter is a shrewd politician who knows the system and how to be successful in it, analysts say. He also became known as one of the Senate's more moderate voices and attached his name to several bipartisan bills.
"You don't survive as long as he has without having political wits about you," Gonzales said.
When it comes to political figures, "no one has traveled the state in modern history more than he has," Madonna said. "No one has done more to bring home the bacon, as they say."
"He's smart, aggressive, does his homework and is relentless as a campaigner," Madonna said. "It just so happened that he met his match in relentlessness."
Madonna said the vote for Obama's stimulus plan was the "crowning blow" in Specter's GOP career, but it wasn't the first time the outspoken senator broke away from his party. In his second term, in 1987, Specter became the only Republican to vote against conservative Supreme Court nominee Robert H. Bork. Through the Reagan years, he also consistently voted against proposals to cut social programs. And when President Clinton's impeachment case was brought to the Senate in 1999, he sided with the Democrats.
One of the reasons Specter lasted so long in the Senate is that he has "a work ethic beyond reason," Madonna said. He's "smart, aggressive, unafraid to get himself into controversies -- from the single bullet theory [claiming that President Kennedy and Texas Governor John Connally were hit by the same bullet in Dallas] to interrogating Anita Hill to this Scottish law impeachment of Clinton."
After criticizing his own party on Clinton's impeachment, Specter cited Scottish law and gave Clinton a verdict of "not proven," a ruling unheard of in the U.S. Congress.
"Arlen has been a dominant political personality for decades. ... He has done his fair share of taking care of business back home. He's been an untiring public servant whether you agree with him or not. He works all the time," Ridge said. "It will be a huge loss to Pennsylvania."