Leahy, third in line to the presidency, told The Associated Press that the US Airways flight landed smoothly and passengers were being transferred to another flight to Burlington.
The pilot reported hydraulic problems soon after leaving Washington's Reagan National Airport at about 10:15 a.m., prompting him to divert to Philadelphia at about 11:30 a.m., officials said.
Flight 3235 landed safely and, following protocol, was towed to the gate, airline spokesman Todd Lehmacher said. The Republic Airlines regional jet carried 80 passengers and four crew members, he said.
Leahy said he'd known the pilot, Drew Chace of Vermont, for years, and said he did "a remarkably good job."
The Federal Aviation Administration was investigating.
Leahy, who has represented Vermont in the Senate since 1975, said he and his wife, Marcelle, fly between Burlington and Washington several times a month and had never had a serious problem. He said he's had more hair-raising experiences flying overseas but would still be especially glad to see his Middlesex home.
"I must admit the front porch in the farmhouse always looks good. Tonight it's going to look twice as good," Leahy said.
The airline's website showed the new flight landed in Burlington at about 2:15 p.m.
Leahy, president pro tem of the senate, had a security team traveling with him.
Associated Press writer Wilson Ring in Montpelier, Vt., contributed to this report.