Congress' annual defense authorization bill, the National Defense Authorization Act (NDAA), is in jeopardy this year as partisan clashes jeopardize its 65-year unbroken record of passage.

The must-pass Pentagon policy measure carries a historic request of $1.15 trillion, rising to $1.5 trillion if the Trump administration secures its third reconciliation bill, a steep climb for lawmakers.

Democrats argue the top-line must be trimmed by hundreds of billions and seek safeguards around the ongoing war with Iran before they can support the bill, setting up a clash with the administration, which refuses to lower the figure or accept a Tehran peace deal that satisfies Democrats.

Defense-budget specialist Mackenzie Eaglen of the American Enterprise Institute said the gap between a "nightmare scenario" where the NDAA fails and a version that includes a third reconciliation bill is the widest she has ever seen.

"The politics are absolutely dismal between the two branches right now," she told The Hill.

Eaglen warned that breaking the streak could make future NDAAs harder to pass, noting we are in a "norm-shattering time of great disruption, including in politics."

"Once red lines are crossed, it is difficult to go backward. I fear the necessary and bipartisan NDAA is fragile in this new era but hope I'm wrong," she added.

The bill's rocky path emerged in June when the House Armed Services Committee approved its version 44-12, with only half of the Democrats on the panel voting for it, unusual for the historically bipartisan committee.

In July, the full House passed the NDAA by a narrow 216-212 margin, a departure from the broad coalitions that usually carry the defense legislation. A small group of Democrats joined Republicans to approve it.

The Senate proved even tougher. In July, a procedural vote of 50-46 fell short of the 60 votes required to move the Senate version forward.

Democrats blocked it over concerns about overall defense and non-defense spending, citing the resumed Iran war and Trump's failure to request congressional authorization for the conflict that began on Feb. 28.

Senate Armed Services Committee Chairman Roger Wicker (R-Miss.) called the Democratic vote to halt the NDAA "unprecedented."

A defense-industry source said Republicans are likely to hold firm on the $1.15 trillion figure, giving Democrats leverage for concessions, yet the bill could still face a Trump veto.

"I think there's a lot of things the Democrats will ask for, including possibly some language on Iran, but I think that it still becomes harder to pass," the source said.

Late this month, House and Senate members began informal talks on a way forward. Staff have finished drafting, and unresolved issues have moved to the staff director level. The "Big Four", the chairs and ranking members of the House and Senate defense committees, will take up the bill after the November midterms, aiming for completion by Thanksgiving.

Another issue is the Pentagon's push to rename the Department of Defense as the "Department of War," with both armed services committees including language in their drafts.

Democrats label the change unnecessary and accuse the administration of semantics.

Other culture-war language may be dropped, while Iran-related provisions could be added.

The Iran war cost has risen to $47 billion, up over $3 billion since last month, as Trump considers strikes after the midterms.

In September, the Senate rejected a House resolution urging Trump to withdraw troops from Iran. Growing GOP dissatisfaction with the president's handling of the war and a potential Democratic takeover in the Senate present a new chance to curb the administration.

Lawmakers appear determined to finish the bill.

"It's very important," House Speaker Mike Johnson (R-La.) told Punchbowl News. "You know, we are in very dangerous times. There's a lot going on. We have to come together and get it done."

Michael Herson, president of lobbying firm American Defense International, said the process will be bumpy, as it has been in recent years, but expects the NDAA to pass and keep its streak.

"I haven't given up hope on that," he added.