Rep. John Dingell (Mich.) isn’t the only senior Democrat nervous about the challenge from Rep. Henry Waxman (Calif.) for the Energy and Commerce Committee gavel.

Several top Democrats backing Dingell argue that the contest, dividing House Democrats this week even as they celebrate their expanded majority, represents an assault on a long-sacred tradition in the Caucus: the seniority system.

Jitters about upending that tradition appear to be benefiting Dingell down the stretch with some of the most prominent members of the Caucus: his fellow chairmen, who are so far breaking for the Michigan lawmaker 6-2. The Democratic Steering Committee considers the issue today, and the full Caucus will vote on it Thursday.

“It’s a part of it — but not the whole thing,” Financial Services Chairman Barney Frank said. The Massachusetts liberal, whose outspoken views put him more ideologically in line with Waxman, announced his support for Dingell this week.

But how that support will translate to a Caucus also stretched by ideological, geographic and issue-based loyalties is unclear, and Democratic insiders said the race still appears too tight to handicap, with only one full day left for the two contenders to make their cases.

“It’s a tough one. It’s too close to call,” Rep. G.K. Butterfield (D-N.C.) said.

For chairmen, this battle could be prelude to a more fundamental clash about their prerogatives. If Dingell is toppled, others could field challenges of their own. That would compound another nettlesome problem that remains on the books: Speaker Nancy Pelosi (D-Calif.) has preserved Republican-authored six-year term limits on chairmen.

“Term limits and the seniority system together are definitely why Dingell is getting some of the support he’s getting from chairmen,” one senior Democratic aide said. “They are generally pushing back on Pelosi and her taking some of their power away from them. … None of them want to feel that if they don’t do things [as] she wants them done, they’ll be at risk of losing their chairmanship.”

Pelosi has taken pains to distance herself from the fight over the Energy and Commerce gavel, but several top Democrats on and off Capitol Hill point out that she has not stepped in to stop it, either.

Looking beyond the Dingell fight, several chairmen said they will press to have the term limits stripped as House Democrats cobble together the rules package for the next Congress.

Rules Chairwoman Louise Slaughter (D-N.Y.) said she included the issue in a list of about two dozen other possible changes she recently sent to Pelosi for review and that it will be up to the Speaker to make the call. In recent months, Pelosi has signaled she is in no hurry to remove the limits.

“We will deal with that issue when we deal with it,” she told Roll Call in a June interview.

Broadly, a senior Democratic leadership aide argued that the Speaker has maintained close working relations with her chairman and kept them at the forefront of debates and negotiations with the Senate and White House.

But quietly, senior Democrats have frequently complained that Pelosi stepped on chairmen’s toes, an abrogation of Democratic tradition.

Democrats have honored the seniority system for generations — rewarding their longest-serving members with gavels, then allowing them to continue amassing power by hanging on to those chairmanships indefinitely.

Several of the Democratic “Old Bulls” now leading committees were reared in that order, and Dingell, who became the top Democrat on the Energy and Commerce panel in 1981, is prime among them.

For these lawmakers, the return to the majority was at times a rude awakening. At key moments over the past two years — in debates on auto efficiency, children’s health insurance and the first economic stimulus package earlier this year, among others —Pelosi repeatedly demonstrated she has no compunction forgoing the regular order Democrats pledged to restore and seizing the initiative from her committee chairmen.

“It’s not the same situation today, and they get that,” one senior aide said. “At the same time, they do have a feeling they’ve earned some power to run their committees the way they see fit.”

How the seniority issue affects the Energy and Commerce contest is not yet clear. So far, it is helping rally support among members of the Congressional Black Caucus, who cite respect for length of service in explaining their support for Dingell.

“Seniority is important. It’s been a part of this institution for a long time. So absent cause, I see no reason to change” chairmen, said Rep. Gregory Meeks (D-N.Y.), a prominent CBCer.

That system has helped reward three CBCers with gavels, and two of them — Ways and Means Chairman Charlie Rangel (D-N.Y.) and Judiciary Chairman John Conyers (D-Mich.) — have endorsed Dingell.

But Waxman’s camp continues to wage a quiet campaign, making it difficult to determine momentum, even at this late stage. Nevertheless, the challenger appears to have locked up the support of one key group: incoming freshmen.

An informal survey of about two dozen incoming Democrats resulted in 18 saying they support Waxman and just three saying they support Dingell, said one freshman lawmaker who requested not to be identified.

The lawmaker is “not surprised” by these results since freshman Democrats were largely elected after campaigning for change.

Waxman has also been more aggressive in reaching out to new Members, the lawmaker said. “Dingell is now trying to spend more time with freshmen.”

Democratic leadership elections went as expected, with Rep. John Larson (Conn.) moving up to Caucus chairman and Rep. Xavier Becerra (Calif.) winning the race for vice chairman over Rep. Marcy Kaptur (Ohio). Speaker Nancy Pelosi (Calif.), Majority Leader Steny Hoyer (Md.), Majority Whip James Clyburn (S.C.) and Democratic Congressional Campaign Committee Chairman Chris Van Hollen (Md.) were unopposed, with Van Hollen adding the title of special assistant to the Speaker.

Steven T. Dennis and Jen Bendery contributed to this report.