The Vermont Democrat's colleagues feted him on the Senate floor -- and Leahy, who was first elected in 1974, used the moment to tout bipartisan accomplishments.
"The Senate, at its very best, can be and should be the conscience of the nation," he said.
He listed some of his most memorable efforts in the Senate: Writing and enacting the Organic Farm Bill; regulating mercury pollution; privacy and civil liberty battles; nutrition bills to help low-income Americans; campaign finance and patent reform; reauthorizing and expanding the Violence Against Women Act; and opposing the war in Iraq.
Leahy, 75, still has ground to make up if he's to set a new record: The all-time vote leader is former Democratic Sen. Robert Byrd of West Virginia, who voted 18,689 times, according to the Senate Historian's Office.
Others who have exceeded 15,000 votes: Daniel Inouye, D-Hawaii; Ted Kennedy, D-Massachusetts; Ted Stevens, R-Alaska; and Strom Thurmond, R-South Carolina.
Just 31 senators have cast more than 10,000 votes -- though several are still active.
Leahy's friend Chuck Grassley, the Iowa Republican senator, is set to reach a milestone of his own -- 12,000 votes -- later on Tuesday.
The two work closely together: Grassley chairs the Senate Judiciary Committee, while Leahy is the panel's top-ranking Democrat.
Other active senators who have topped 10,000 votes but are still working toward 15,000 are Thad Cochran, R-Mississippi; Orrin Hatch, R-Utah; and the Majority Leader, Mitch McConnell, R-Kentucky.