I always thought it was the other way around, but then I read this (Letter 246):
"Of the others only Gandalf might be expected to master him – being an emissary of the Powers and a creature of the same order, an immortal spirit taking a visible physical form. In the 'Mirror of Galadriel', 1381, it appears that Galadriel conceived of herself as capable of wielding the Ring and supplanting the Dark Lord. If so, so also were the other guardians of the Three, especially Elrond. But this is another matter. It was part of the essential deceit of the Ring to fill minds with imaginations of supreme power."
This "especially Elrond" confuses me. I don´t think that Elrond is inherently more powerful than Galadriel, who is described as the greatest of the Noldor, except for Fëanor (and possibly even his equal).
A quote from Unfinished Tales says:
"...In this he (Tolkien) emphasized the commanding stature of Galadriel already in Valinor, the equal if unlike endowments of Fëanor; and it is said here that so far from joining in Fëanor's revolt she was in every way opposed to him."
"A queen she was of the woodland Elves, the wife of Celeborn of Doriath, yet she herself was of the Noldor and remember the Day before days in Valinor, and she was the mightiest and fairest (so even fairer than Arwen I would guess) of all the Elves that remained in Middle-earth."
(The Silmarillion, "Of the Rings of Power")
"Galadriel was the greatest of the Noldor, except Fëanor maybe, though she was wiser than he, and her wisdom increased with the long years."
(The Peoples of Middle-earth, "The Shibboleth of Fëanor")
"These two kinsfolk (Fëanor and Galadriel), the greatest of the Eldar in Valinor, were unfriends for ever." "Who together with the greatest of all the Eldar, Luthien Tinuviel, daughter of Elu Thingol, are the chief matter of the legends and histories of the Elves."
The latter means that Luthien, Fëanor and Galadriel are the chief matter of the history and legends. It seems to me that Tolkien thought of those three as the crème-de-la-crème of the elven crop.
Reading all that I have serious problems believing that Elrond could be superior to her. As great as Elrond is, descending from Melian, he is no equal of Fëanor, who is said to be the mightiest of all elves, and if Galadriel is his equal, then there is no way that Elrond would be mightier than her.
Maybe I just read this "especially Elrond" wrong; it could as well mean that especially Elrond was deceived into thinking that he could master the ring, because that was the deceit of the ring. And Tolkien said explicitly that only Gandalf could possibly master it.
So, was Elrond more powerful than Galadriel?