The Democratic Party primary for the 2010 United States Senate election in Pennsylvania took place on May 18, 2010, when Congressman Joe Sestak defeated incumbent Arlen Specter, which led to the end of Specter's five-term Senatorial career. Just before the start of the primary campaign, after serving in the Senate as a Republican for 29 years, Specter had switched to the Democratic Party in anticipation of a difficult primary challenge by Pat Toomey; Toomey ultimately defeated Sestak in the general election. Political observers and journalists described the race between Specter and Sestak as one of the bitterest and most-watched of all the 2010 primary elections.
Although Sestak was initially urged to run by the Democratic establishment, Specter gained broad support from Democrats after he switched parties. Prominent political figures like President Barack Obama and Pennsylvania governor Ed Rendell later tried to sway Sestak from continuing the race, fearing he would damage Specter's chances in the general election. Former President Bill Clinton offered Sestak a position in the Obama administration if he withdrew his candidacy, an offer Republicans would later criticize. Sestak refused to drop out and criticized Specter's party switch as an opportunistic move aimed solely at self-preservation. Nevertheless, Sestak struggled to overcome problems stemming from low name recognition and Specter's support from such individuals as Joe Biden and Harry Reid, and organizations like the AFL–CIO and Pennsylvania Democratic Committee.
Specter led Sestak by more than 20 percentage points for most of the race. However, this lead narrowed significantly in the final month of the campaign, when Sestak concentrated his funds and efforts on television commercials questioning Specter's Democratic credentials. Specter grew more critical of Sestak as the race progressed, attacking his House attendance record, accusing him of failing to pay his staffers minimum wage and alleging he was demoted while serving in the U.S. Navy for creating a poor command climate. Political observers said Sestak's commercials played a major part in his victory in the primary election. A national swing in momentum toward Republicans and against incumbents ultimately harmed Specter's chances. As of 2026, it is the last time where an incumbent Democratic senator lost renomination in a primary.
Background
Chris Matthews speculation
Beginning in April 2008, the media reported growing speculation that Chris Matthews, news commentator and host of MSNBC's Hardball with Chris Matthews, might run in the 2010 Democratic primary for the United States Senate Pennsylvania seat then occupied by Republican Arlen Specter. In an interview with The New York Times Magazine, Matthews said that he believed Specter had been in the Senate for too long, but that running for Senate would mean giving up a career he loved. Mark Leibovich, author of the article, wrote, "Matthews has been particularly obsessed with Pennsylvania of late, devoting hours on and off the air to the state's upcoming Democratic primary, staying in close contact with the state's party apparatus". Speculation was further fueled by Matthews' appearance on an April 14 episode of The Colbert Report. Host Stephen Colbert asked Matthews about the rumors and prodded him to make a public announcement, to which Matthews replied, "Did you ever want to be something your whole life? ... When you grow up, some kids want to be a fireman. I want to be a Senator. But I have to deal with these things as they come." Matthews declined to directly answer questions about his possible candidacy when pressed by the media, but did not deny the possibility.
The subject of Matthews' possible candidacy was raised at an October dinner fundraiser for presidential candidate Barack Obama held by Robert Wolf, president of UBS's investment bank. Discussing the dinner later, Matthews told The New York Times, "People have asked me about it. I've never told anyone that I'm running." On November 28, The Patriot-News of Harrisburg reported that Matthews met that week with Pennsylvania Democratic Party Chairman T.J. Rooney and Executive Director Mary Isenhour to discuss possibly running against Specter. Isenhour told the paper Matthews had not formed a campaign committee or begun raising money, and she did not believe he had yet come to a firm decision, adding, "He's got a really good job with MSNBC. I think he's going to put some thought into it before he jumps in." The same day, the political blog FiveThirtyEight.com reported that Matthews had already met with potential campaign staff, something Matthews claimed was "absolutely not true".
The increased speculation led some, like former spokesman for Hillary Clinton's 2008 presidential campaign Phil Singer, to criticize Matthews for openly weighing a political campaign bid while working as a news broadcaster. Singer believed Matthews should resign or be suspended from the network until a decision was made. Speculation grew as Matthews spent much of 2008 attending meetings with Pennsylvania representatives and major Democratic fundraisers. Matthews discussed a possible campaign with Pennsylvania Governor Ed Rendell, and poll numbers for a theoretical race showed him only three percentage points behind Specter. However, the speculation came to an end on January 7, 2009, when Matthews told his Hardball staff he was not going to run for Senate. Such media outlets as The New York Times and The Washington Post suggested Matthews' alleged consideration was a tactic to ensure a higher salary during negotiations with MSNBC to renew his contract, which was set to expire in June 2009.
Arlen Specter party switch
As early as 2008, five-term Republican Pennsylvania Senator Arlen Specter had made clear his plans to seek re-election. Specter had narrowly avoided a primary defeat against conservative challenger Pat Toomey during his 2004 Senate race, and he was expected to face an even greater challenge from Toomey in 2010, particularly since the conservative faction of the Pennsylvania Republican Party had vowed to defeat Specter in the upcoming primary. Some high-profile Democrats, including Vice President Joe Biden and Governor Rendell, began encouraging Specter to join the Democratic Party by publicly offering to help Specter raise money if he switched. A March 2009 Quinnipiac University Polling Institute poll found Specter trailing Toomey 41 percent to 27 percent among Republican primary voters, in large part due to voter angst over Specter's support for the American Recovery and Reinvestment Act of 2009, which was supported by President Obama. Since the Pennsylvania primaries are closed, the poll noted that Specter could not be assisted by support from moderates or Democrats. Clay F. Richards, assistant director of the Quinnipiac University Polling Institute, said of the poll results, "Pennsylvania Republicans are so unhappy with Sen. Specter’s vote for President Barack Obama's Stimulus Package and so-called pork barrel spending that they are voting for a former Congressman they hardly know."
A March 2009 article in The Hill quoted Specter as stating he was considering leaving the Republican party to run for re-election as an Independent candidate. Specter later denied the validity of those claims, announcing on March 18, "To eliminate any doubt, I am a Republican, and I am running for reelection in 2010 as a Republican on the Republican ticket." However, on April 28, 2009, Specter announced he was leaving the Republican Party and becoming a Democrat because he disagreed with the increasingly conservative direction the Republican Party was heading in and found his personal philosophy was now better aligned with the Democrats. Although Specter said that he primarily based his decision on principle, he also admitted it was partially due to his poor chances at winning the Republican primary: "I have traveled the state and surveyed the sentiments of the Republican Party in Pennsylvania and public opinion polls, observed other public opinion polls and have found that the prospects for winning a Republican primary are bleak."
Prior to switching parties, Specter said he had been assured by Senate Majority Leader Harry Reid that he would keep his seniority on the Senate if he joined the Democratic Party. The arrangement displeased some Senate Democrats, and, on May 3, the Senate voted to strip him of his seniority in spite of this. This temporarily made Specter the most junior Democrat in the Senate and severely limited his influence as a legislator. Nevertheless, Specter's decision was praised by many major Democrats, including Reid and President Barack Obama, who promised to campaign for him. Republicans, however, criticized the decision, accusing Specter of betraying his principles and party to preserve his political career. Michael Steele, chairman of the Republican National Committee, said Specter "flipped the bird" to the party and that Specter made the decision based solely because he knew he would lose the election. Senator John Cornyn, chairman of the National Republican Senatorial Committee and a past supporter of Specter, said the decision "represents the height of political self-preservation". Specter defended his position by arguing the Republican Party had strayed too far from the vision of President Ronald Reagan, adding, "I am not prepared to have my 29-year record in the United States Senate decided by the Pennsylvania Republican primary electorate."
Joe Sestak declares candidacy Second-term U.S. Representative Joe Sestak, a former U.S. Navy admiral, began privately discussing the possibility of seeking the Democratic nomination with his family in December 2008. Senator Bob Menendez, chairman of the Democratic Senatorial Campaign Committee, approached Sestak in April 2009 and asked him to run, but Sestak claimed he was not initially interested. Nevertheless, media speculation about Sestak's possible campaign began as early as mid-April 2009, and intensified the day Specter changed political parties. While most Democrats embraced the long-time Senator, Sestak issued a statement criticizing Specter's decision, declaring it an opportunist move that should have been made in consultation with Pennsylvanians rather than the Senate Democratic leadership and the Washington political establishment. Menendez approached Sestak again, this time asking him not to run against Specter, but Sestak did not agree to back down. Later, when asked by Fox News, Sestak insisted he had not decided whether he would run for the office, but declined to immediately endorse Specter and said he had to "wait and see". During a May 3 appearance on CNN's State of the Union with John King, Sestak questioned whether Specter was really a genuine Democrat, adding, "I think Arlen has to tell us not that it was too hard to run against someone. ... What I need to know is, what is he running for?"