By Brian Rosenwald

Sen. Pat Toomey has won plaudits for his moderation, and to some he might appear to be the latest in Pennsylvania's proud lineage of moderate Republicans senators.

But in Toomey's case, the label belies his record. When assessed by Common Space DW-Nominate scores, a scholarly metric that places legislators' voting records along an axis - negative 1 being the farthest left and 1 the farthest right, though Common Space does not assign labels - Toomey amassed the most conservative voting record (.643) of the nine GOP senators elected from Pennsylvania between 1930 and today.

This result might surprise observers, because the pugnacious Rick Santorum employed far more incendiary conservative rhetoric than the mild-mannered Toomey. Yet Santorum compiled a significantly more moderate voting record - with a Common Space score of .339.

The mislabeling of Toomey illustrates both how meaningless the term moderate is today and how far to the right the Republican Party has shifted over the past few decades.

The perception of Toomey as a moderate stems from his quiet and thoughtful demeanor, as well as his cosponsorship of several measures with Democrats, including a gun-control proposal with Sen. Joe Manchin (D., W.Va.) that required standing against the vast majority of his Republican colleagues. Toomey has even earned the admiration of several major Democratic donors who hosted fund-raisers for him.

One of them, Alan Kessler, told the Inquirer that we "need more Pat Toomeys." Kessler argued that, "time and time again, he has reached across the aisle to work with Democrats and do what people are looking for: someone who goes to Washington and gets things done."

Yet in actuality, the election of Santorum and Toomey signified the end of the era in which the Pennsylvania GOP firmly occupied the political center with great electoral success.

In the Senate, James Duffy's 1950 election ushered in this era, in which commonwealth Republicans would successfully elect five moderate senators (all with Common Space scores of less than .158).

Four of these senators served at least two terms, with Hugh Scott occupying the post of Senate Republican leader and Richard Schweiker later serving as health and human services secretary under President Ronald Reagan.

Arlen Specter had the lengthiest tenure, winning five statewide elections. In 2004, President George W. Bush and Santorum helped Specter withstand a primary challenge from then-Congressman Toomey.

However, Specter's decision to support President Obama's stimulus plan, which embodied his frequent willingness to infuriate both left and right, rendered him toxic to Republican voters. Facing certain primary defeat, the former Democrat returned to the party of his youth in 2009.

The notion of Toomey as a moderate, therefore, illustrates just how far the Pennsylvania GOP has departed from its centrist roots and how rapidly the definition of moderate has changed.

Unwittingly, Specter may have contributed to this transformation, which spelled his eventual downfall.

In 1994, then-Congressman Santorum challenged Sen. Harris Wofford (D.) for the seat to which he was appointed after the tragic death of John Heinz. Specter had ardently courted prominent moderates to oppose Wofford, including state Auditor General Barbara Hafer and Heinz's widow, Teresa. When they refused, however, Specter helped Santorum raise funds, and several of his staffers joined Santorum's campaign.

Specter's prioritization of party unity in 1994 epitomized the proclivities of Republican moderates. This behavior provides clues to one reason that conservatives have thrived in the GOP over the last 40 years while moderates have drifted toward extinction.

Except in the rarest cases (such as Barry Goldwater's candidacy in 1964), moderates support conservative nominees against even the most moderate Democrats. While Specter preferred a centrist nominee in 1994, when none emerged, he labored for Santorum. Centrists also almost never challenge incumbent conservatives in primaries.

By contrast, conservatives only sometimes display this level of party loyalty. Along with media allies and conservative groups like the Club for Growth (which Toomey previously helmed), they frequently target sitting moderates in primaries.

In 2004, Specter barely repelled Toomey's challenge, and in 2010 and 2012, conservatives upended moderate or establishment Republicans in Senate primaries in Delaware, Alaska, and Indiana. By contrast, in neither 2000 nor 2006 did Santorum face a primary challenger.

The two sides are playing fundamentally different games. As a result, today's Republican "moderate" is actually a conservative maverick willing to depart from orthodoxy on occasion while refusing to vilify the opposition - and the old-fashioned centrists of yesteryear are virtually extinct.

If centrists ever wish to reemerge in the Republican coalition, they have to emulate conservatives' tactics. That means challenging conservatives in primaries in moderate places and sometimes refusing to endorse them, or even supporting moderate Democrats. They must also build networks to champion true centrists.

Only these steps will successfully rebuild the centrist wing of the GOP.

Brian Rosenwald is an alumni research and service fellow at the University of Pennsylvania's Robert A. Fox Leadership Program.