Kennedy said he thought the compressed debate on Wednesday evening over an amendment to a defense appropriations bill was not the right venue for consideration of the issue, "pitting national security against civil liberties."

US Representative Joseph P. Kennedy III, the only member of the Massachusetts House delegation to vote against legislation curtailing the government's data snooping power, said Thursday he would be "adamant" in advocating for increased congressional oversight of such programs.

The Brookline Democrat said he opposed the amendment because, after meetings with intelligence officials, he concluded that the National Security Agency's previously secret initiative to track hundreds of millions of Americans' phone records had prevented terrorism.


"As I went around and went through those briefings and looked carefully at the system that was set up, at both the process and the procedure, I came to the decision that this is a program that has helped thwart terrorist attacks both here and abroad," Kennedy said.

The amendment, which was defeated on a 217 to 205 vote that forged coalitions between libertarian Republicans and liberal Democrats and between leadership Republicans and more security-minded Democrats, would have stripped the NSA of its statutory authority to collect data from people not under investigation. Under the Patriot Act, the NSA mines domestic phone records and metadata.

The surveillance techniques came to light last month when Edward Snowden, a former NSA contractor, leaked details to the press. Snowden has applied for political asylum in Russia, where he is a fugitive.

The Bay State's seven other House members voted to support the amendment, sponsored by Representative Justin Amash, a Michigan Republican. A ninth seat has been vacant since Edward J. Markey resigned to become a senator.

In all, 83 Democrats and 134 Republicans opposed the measure, which sparked an unusually lively debate between members of the same party.


Proponents depicted the measure as a defense of privacy in the face of a far-reaching security state, while opponents called it a dangerous lowering of the nation's guard against terrorist threats.

Kennedy said the government needs to be "far more vigilant about striking the right balance between national security and privacy."

He said he planned to "push for this debate into the fall to make sure that adequate reforms are put in place."

The vote marked the first time Kennedy had peeled off from his Bay State colleagues in the House on a closely watched vote. Kennedy has also been more outspoken in favor of escalated US support for rebels in Syria.

And, given his status as the latest heir in a political dynasty that has both controlled and been on the targeted side of the intelligence apparatus, Kennedy's choice was sure to receive added attention. His great-uncle President John F. Kennedy feuded with the Central Intelligence Agency but also used it as a tool in the Cold War. Both President Kennedy and his brother, Attorney General Robert F. Kennedy, had testy relationships with J. Edgar Hoover, director of the Federal Bureau of Investigation.

Another of Representative Kennedy's great-uncles, Senator Edward M. Kennedy, so worried President Nixon with his political potential that Nixon instructed aides to wiretap him and suggested that the Internal Revenue Service should investigate his tax returns.

Asked whether his family's history factored into his voting calculus, Kennedy deflected the question, pivoting to his own experience as an assistant district attorney.


"I think really what influenced me more than probably anything else was my own experience as a prosecutor and looking at our criminal law system about what measures do you have to take to get criminal warrants," he said.

Kennedy said he had consulted with other Massachusetts members, referring to the group as "a close delegation." In the end, though, the state's youngest member went his own way.

In a statement released Wednesday, Representative Niki Tsongas, a Lowell Democrat, said, "The bipartisan support for this amendment makes clear that there is skepticism on both sides of the aisle about the wide-ranging actions taken by the NSA, and sends the administration a compelling message that protecting the privacy of our citizens in this digital age is ever more essential."

O'Sullivan can be reached at James.OSullivan@globe.com. Follow him on Twitter at @JOSreports.