The phrase "argued relentlessly" is correct and usable in written English.
It can be used to describe someone who is persistently and tirelessly making a case or defending a position in a discussion or debate.
Example: "During the meeting, she argued relentlessly for the need to increase the budget for the project, highlighting its potential benefits."
Alternatives: "debated tirelessly" or "contended persistently".
He has always argued relentlessly that the charges of racism in South Boston were overstated.
She generally favors moderates but has given to several white-hot partisans (including Representative Dan Burton, a Clinton foe who argued, relentlessly, that the White House aide Vincent W. Foster Jr. had been murdered, and who memorably re-enacted the shooting in his backyard).
He argued relentlessly over decades for the adoption of the minimum wage.
We've argued relentlessly that I need to be able to compare my student's reading ability with the reading ability of Chris McNoname in Iowa, so why can't I compare the tests that each one is taking?
His nephew, Robert F. Kennedy Jr. -- himself an environmental attorney and activist and founder of the watershed protection group Waterkeeper Alliance -- has argued relentlessly against the project, including in prominent op-eds in The New York Times and, more recently, The Wall Street Journal.
Save when at work making distinctions and arguing relentlessly, philosophers are much like everyone else.
It puts human faces behind the topics we argue relentlessly on Facebook.
Indeed, Israel and the United States argue relentlessly that a Palestinian state can only exist as the result of a negotiated "solution" acceptable to Israel.
The labels fully understand that recorded music, streamed or downloaded, is going to be free in the future (we've argued this relentlessly).
Yet I've been arguing in relentlessly optimistic speeches and articles that everyone is entitled to a second chance in life.
The wink-and-a-nod banter culture tells the minority of self-entitled bullies, like, for instance, the male (former) friend of mine - incidentally, no oil portrait himself – who argued with me, relentlessly, for years after I dated a size sixteen woman, insisting that she was unattractive, and I was wrong to disagree, and I shouldn't have gone out with her.
