The fight for one of Rep. Edward J. Markey’s plum committee slots may come down to whether issue and identity politics trump the time-honored seniority system.

Reps. Peter A. DeFazio of Oregon and Raúl M. Grijalva of Arizona both want to be the top Democrat on the House Natural Resources Committee — assuming Markey wins his Massachusetts Senate bid in the June 25 special election to succeed John Kerry, who left the post to become secretary of State.

DeFazio is the panel’s second-most-senior Democrat, and perhaps his biggest selling point — next to his legislative record — is his seniority.

It’s something many of DeFazio’s peers are particularly deferential about, and it has led to a large number of lawmakers flocking to his side of the field.

Last month, 20 self-identified progressive members of the House Democratic Caucus signed a letter supporting him.

“Peter has the seniority, the experience, the expertise, and the tenacity to challenge the Republicans’ ideological rhetoric,” they wrote. “He has ably served on the committee for 26 years — serving on every subcommittee — and mastered the wide range of natural resources issues confronting the committee.”

While DeFazio may be scoring inside endorsements, Grijalva is trying to rack up outside backing to help his bid.

Though many of the mainstream national environmental groups haven’t yet weighed in on the campaign, some organizations see the significance of elevating the Congressional Progressive Caucus co-chairman to a higher seat at the Democratic leadership’s table. And they are starting to speak up.

Mark Magaña, executive director of the National Latino Coalition on Climate Change, told CQ Roll Call that his organization is considering sending a support letter for Grijalva signed by its member organizations.

While Grijalva’s environmental record is integral to the coalition’s support, Magaña said, the congressman’s ascent to the ranking member slot would mean there would be another pro-immigration Democrat on the committee leadership roster.

“We need to increase that number, and this would be an important opportunity to do that,” he said.

To be sure, Magaña said his group couldn’t support Grijalva’s candidacy if he didn’t have an “exemplary” voting record on pollution and conservation issues. But it’s also important for Democratic leaders to consider where candidates stand on issues important to the Hispanic community, he said. DeFazio’s past votes on immigration — particularly his vote for the 2005 immigration overhaul backed by President George W. Bush that never became law — are worrisome, he said.

“We want to make sure that people who took that vote in a negative way are held accountable,” Magaña said.

Before voting for the 2005 GOP-backed bill, DeFazio noted in a floor speech that he was not enamored of the entire measure. “Immigrant rights organizations, labor unions and others have rightfully pointed out that the bill does not in any way address the 10-12 million undocumented workers already in the U.S., no matter how long they’ve been here or how much they’ve contributed to their local community or the economy,” he said at the time. He added he was voting for it because it was the only bill that allowed him to support enhanced border security.

Another organization is backing Grijalva because of local ties. The Center for Biological Diversity has its headquarters in Grijalva’s hometown of Tucson, Ariz. The group has gotten to know him through local environmental issues over the years and has worked with him on wilderness and endangered species issues, said Bill Snape, the group’s senior counsel.

But while Grijalva and DeFazio are challenging each other’s records, Snape’s organization is keeping its support for Grijalva completely focused on what he could bring to the position, rather than denouncing DeFazio. “We acknowledge there are other good candidates,” Snape said.

The Center for Biological Diversity was, incidentally, also one of hundreds of environmental organizations that urged President Barack Obama to nominate Grijalva to lead the Interior Department in his second term.

Grijalva might also get support from Native American groups as well as organizations promoting stricter gun control. In a letter to colleagues Thursday, Grijalva slammed DeFazio for, among other things, “allowing guns into national parks, which I opposed and which he voted to allow as an amendment to an unrelated credit card reform bill.”

That issue, which came up in May 2009, was fought by the Brady Campaign to Prevent Gun Violence, among others.

Grijalva said Thursday that while he would be “heartened” by outside organizations speaking up on his behalf, he couldn’t say what influence they could have among his colleagues. But he said he hoped he would be seen as “a bridge” for different constituencies in the party.

“Quite honestly, there is about 191 members of Congress who make this call and what outside opinion does to them or not, it’s in the eye of the beholder,” he said.

Grijalva added that he, too, had supporters within membership ranks, but is keeping those names to himself. One Democrat who has spoken in his favor is Congressional Hispanic Caucus Chairman Rubén Hinojosa of Texas.

DeFazio said in a statement that while he has “spoken with many major, national conservation and environmental groups … [I] am not aware of any interest in getting involved. … Mr. Grijalva and I have very similar environmental records and the groups with interest in the committee have no reason to pick between two of their friends since they ultimately don’t get to vote,” he said.