In 1946, Markey ran a closely contested but unsuccessful campaign for one of Maryland's U.S. Senate seats as a Republican against former Governor Herbert O'Conor. After the vote, both candidates claimed victory, before the official count declared O'Conor the winner by a margin of 2,232 out of more than 470,000 votes cast. On December 10, 1946, Markey requested the Special Committee to Investigate Senatorial Campaign Expenditures conduct a recount in Baltimore City and Montgomery County, which had used voting machines. He asked for a prompt recount and also alleged his opponents' campaign had committed financing violations. The committee agreed because Maryland was unable to conduct its own official recount, and found a variation of about 400 votes. The committee then sought to survey five additional counties that were likely to have irregularities. Markey requested a full recount of the entire state.