The phrase "a longstanding source of" is correct and usable in written English.
It can be used to describe something that has been a consistent or enduring origin of a particular issue, idea, or resource over a significant period of time.
Example: "The river has been a longstanding source of inspiration for local artists, influencing their work for generations."
Alternatives: "an enduring source of" or "a persistent source of".
Piracy is a longstanding source of tension between the music industry and Google.
Paying more money without getting better care in return has been a longstanding source of frustration for employers.
Such disputes are an extension of a longstanding source of tension — sometimes light, other times more antagonistic — between drivers and their self-appointed watchdogs.
ASK a well-informed rider to name the venerable motorcycle maker whose gutsy air-cooled V-twins are a longstanding source of national pride and you might get this surprising, but correct, answer: Moto Guzzi.
The trust is a longstanding source of pride for people who call Hershey home and revere its benevolent capitalist founder but in recent years it has become a subject of controversy and intrigue.
Last month, the city and the Port Authority of New York and New Jersey, which owns the site, resolved a longstanding source of tension by giving Mr. Kelly and the Police Department broad control over security at the complex.
Majestic Casual, a longstanding source for deep house edits and superfluous soft-filtered images of women, has just suffered a major blow with the termination of its hugely popular YouTube channel.
Mr. Lapid could push a new government in directions that would ease longstanding sources of tension with Mr. Obama.
In their spare public comments, the two leaders glossed over longstanding sources of friction.
The various attacks undertaken by Qaeda and its affiliates since the occupation began have taken place in countries that are longstanding sources of Osama bin Laden's enmity (like Saudi Arabia) or where an opportunity has presented itself (the suicide bombings in Morocco in May, Indonesia in August and Turkey in November).
Mr. Trump attacked the article, falsely asserting on Twitter that Times reporters relied on "no legitimate sources," a longstanding tactic of his to try to dismiss news reports that portray him negatively.
