U.S. Senator Arlen Specter is under an aggressive assault from his party's activist conservative wing as he attempts to win an unprecedented fifth term as a Pennsylvania Senator.  Stories in Human Events, the National Review, the Weekly Standard, and the Wall Street Journal in varying degrees have raised the prospect that Specter is vulnerable.  His detractors have been motivated by the strong candidacy of Lehigh Valley Congressman Pat Toomey.  Toomey's challenge on the surface appears to be the Senator's most serious primary opponent since his initial election to the Senate in 1980.  

Toomey has for the better part of a year waged an aggressive campaign in spite of the support Specter has received from a wide range of Republican leaders, including President Bush, Vice President Cheyney, U.S. Senator Rick Santorum, and virtually the entire Republican establishment in Pennsylvania, conservatives and moderates alike.  The Toomey-Specter race pits much of the Republican leadership against many of the party's most committed conservative activists. Included among Toomey's endorsers are 23 of the State House's most conservative legislators. This internal struggle is essentially a conflict between pragmatism and ideology.  

The Republican leadership expresses the fundamental pragmatism of a party that desires to retain the seat, notwithstanding their ideological differences with Specter.  Given Specter's campaign skills, public record, and standing in the polls, he is the favorite to win both the primary and general election.  Backing a Republican challenger, even if he is more likely to vote the party line, raises uncertainty about whether the Republicans can hold onto the seat.  The Republicans have a slender one-seat majority in the Senate, and a Specter primary victory gives them the best opportunity to keep the seat.  

The one-vote Republican majority in the Senate makes the reelection of Specter, perhaps, the most critical Republican campaign next year.  Specter may be more independent than most Republicans may be, but he is still a Republican, and he provides the President with his best chance to maintain control over the Senate.  Moreover, Specter has served as the bridge between Senate Republican conservatives and moderates, and on more than one occasion has assumed tough assignments at the request of Republican leaders, and on important procedural votes, Specter is usually with his party's leadership.  Even less well known is the pivotal role Specter played in the election of Rick Santorum in 1994, at a time when Santorum's conservative positions on many issues were much more at odds with Specter's than they are now.

On the other side are those who desire a more ideologically pure candidate.  These Republicans have a visceral dislike of Specter because he is more pragmatic than conservative, and because historically among Republican Senators he is always high on the list of Republicans voting with the Democrats and against the President of his own party.  This dislike has boiled over as evidenced in the recent National Review announcement that Specter is the worst Republican U.S. Senator.  The National Review pulled no punches when it said, "The choice for conservatives is whether to make another uneasy peace with Specter in the prudential belief that no party holding a one-seat majority in the Senate should dump an incumbent who has won four previous elections in a swing state.  The alternative is to rally behind Pat Toomey, an impressive congressman from Allentown who has launched an energetic primary bid against the man who has done more to frustrate conservative goals over the years than perhaps any other member of his caucus."  

Given the hyperactivity of the state's more conservative activists, does Specter have anything to worry about from the Toomey challenge? Historically, Specter encountered conservative opposition in 1986, 1992, and 1998, and he dispatched them with relative ease, despite the lore that he has trouble winning against a variety of weak conservative primary challengers.  In 1986, 1992 and 1998 he won his primary with 76%, 65%, and 67%, respectively, over a variety of conservative and other opponents. 

 Conventional wisdom holds that Pennsylvania voters in both parties prefer moderate candidates in statewide elections.  Pennsylvanian's have, indeed, a long history of electing moderate politicians, such as Hugh Scott, John Heinz, Dick Thornburgh, Bob Casey, Tom Ridge, and Ed Rendell.  Other than Rick Santorum, not a single conservative has been elected Governor or Senator in modern history.  Nevertheless, Santorum was elected in 1994 and re-elected in 2000, and he's a bona fide conservative ideologue.

We have analyzed Keystone Poll data to identify the potential strength of conservative voters within Pennsylvania's Republican Party.  Table 1 displays these key characteristics.  Self-identified conservatives are the most prominent ideological component among the state's registered Republicans, with about half indicating they are conservative.  About one in three say they are moderate and only one in ten says they are liberal.