Rep. Edward J. Markey, D-Mass., has yet to win the Senate seat vacated by now-Secretary of State John F. Kerry, but Reps. Peter A. Defazio and Raúl M. Grijalva are already fighting over who should replace him as the top Democrat on the House Natural Resources Committee.

The two Democratic lawmakers are arguing not only over legislative records and policy positions, but also over the weight that should be given to seniority.

On Tuesday, DeFazio, of Oregon, sent out a “Dear Colleague” letter laying out his credentials and indicating that Grijalva, of Arizona, was lacking in his bona fides — and experience.

“A number of Members I have talked with have expressed some confusion over the seniority of members serving on the committee,” DeFazio wrote in the letter, obtained by CQ Roll Call. “I have served on the Natural Resources Committee for 26 years; Rep. Grijalva has served on the committee for 10 years.”

DeFazio went on to list six members who fall beneath Markey by length in tenure: He is No. 2, whereas Grijalva is No. 7.

“I understand that seniority is not everything in selecting a new Ranking Member,” DeFazio continued. “In a few cases our caucus has chosen a less senior member to lead a committee, but in those few cases the elevated committee member was only one or two slots less in seniority. Rep. Grijalva is seeking to jump over five members with more seniority (all of whom support me). That is unprecedented.”

On Thursday — in his own letter to House Democrats — Grijalva fired back, saying that the more-senior Democrats on the Natural Resources committee have no interest in ascending to a leadership rank.

“Two of them, Reps. Rush Holt and Frank Pallone, are pursuing Senate campaigns,” he wrote. “Rep. Grace Napolitano has said she is not interested, and Rep. Eni Faleomavaega declined the opportunity. I value each of their contributions  . . .  and would gladly have stepped aside had any of them come forward to announce their own candidacy. They did not.

“With all due respect to Rep. DeFazio, I do not believe he is entitled to the position by sole virtue of his seniority,” Grijalva continued. “His letter touts his lifetime League of Conservation Voters ranking of 89 percent as a defining credential. I invite you to compare it to my 96 percent lifetime rating.”

Grijalva also drew comparisons to his support of “cap-and-trade” legislation and the 2011 bill to restore the Gulf Cost — DeFazio opposed both measures.

“Unfortunately, to some extent it’s an uphill battle,” Grijalva reiterated to CQ Roll Call on Thursday afternoon. “The prevailing view, unfortunately, is that I’m somehow, in a very aggressive way, leaping over people in seniority, which is simple not true.”

Though they have for at least a month been lobbying colleagues and outside groups for support, the competition is heating up: the June 25 special election date nears and, if Markey wins, Democrats will want to name a committee successor quickly.