On November 20, 1839, a special election was held in Pennsylvania's 14th congressional district to fill a vacancy caused by the death of William W. Potter (D) on October 28 of that year, before the start of the first session of the 26th Congress.
Election results
McCulloch took his seat on December 2, 1839, at the start of the 1st session of the 26th Congress.
References
Pennsylvania's 14th congressional district special election, 1839 Wikipedia(Text) CC BY-SA