Including yesterday’s appointment of Jeffrey Chiesa, there have been 21 gubernatorial appointments to fill U.S. Senate seats since 1993 — nine resulting from deaths and 12 from resignations. So how does the New Jerseyan fit into the overall pattern?

In 18 of the 20 appointments before Chiesa, the newly named Senators were of the same party as their predecessors. So replacing an archliberal Democrat with a self-described conservative Republican, as is happening in New Jersey, is a real break in usual practice.

However, this is not particularly hard to explain. In only 3 of the 20 cases of vacancies were the Governor and the outgoing Senator of different parties, as with Chris Christie and Frank Lautenberg.

Chiesa fits more comfortably into another emerging pattern: he is a “placeholder” Senator who indicates that he will not run for the seat and who is not really a political figure in his own right. (Although Chiesa was the sitting state Attorney General, New Jersey is one of seven states that fills the AG job by means other than popular election.) Of the 20 other Senators appointed since 1993, seven broadly fit into the placeholder category, with six of these having been appointed just since 2009.

Of the 11 “non-placeholders” who have thus far come up for reelection since 1993, 10 have run to hold their seats and 8 have succeeded — demonstrating the considerable advantages of incumbency. For example, Republican Senators Dean Heller of Nevada and Lisa Murkowski of Alaska pulled off tough re-elections in part due to the advantages of incumbency gained through their initial appointments.

Conversely, placeholder Senators have probably contributed to losses for their parties. Massachusetts Governor Deval Patrick’s decision to fill Ted Kennedy’s seat with a placeholder is clearly linked to the Democrats loss in the subsequent special election. Likewise, Illinois Governor Rod Blagojevich’s choice of the Roland Burris (a complicated affair in its own right) helps to explain the Democrats’ loss of Barack Obama’s seat.

Much ink has been spilled over the past few days concerning Christie’s electoral calculations regarding his upcoming gubernatorial race and beyond. And while Republicans already faced an uphill battle in the upcoming New Jersey Senate race, it’s clear that Christie’s decision to appoint a placeholder does them no favors.

Raymond A. Smith

Raymond A. Smith a Senior Fellow at the Progressive Policy Institute, teaches political science at Columbia and NYU and is an investigator in the Division of Gender, Sexuality, and Health at the Columbia University Medical Center. He is the author of Importing Democracy: Ideas from Around the World to Reform and Revitalize American Politics and Government and editor of The Politics of Sexuality.