Rank-and-file senators are eyeing the annual defense bill the Senate will take up this week as a chance to challenge President Trump's recent controversial moves on national security — but thus far, Republican leaders have resisted their efforts.

Senators of both parties are drafting amendments that would step up sanctions against North Korea, roll back Trump's order to ban transgender troops from the military and force Congress to vote within six months on a replacement authorization for use of military force, or AUMF, against extremist groups. Each of those initiatives grapples with an international crisis that Trump has tackled — with mixed reviews — in the past several weeks, from committing troops to combat in Afghanistan to promising "fire and fury" against North Korea.

But the defense bill's gatekeeper, Senate Armed Services Committee Chairman John McCain (R-Ariz.), has not promised support to any of the amendments and has been unclear about whether he will even allow the measures a vote on the floor.

The behemoth annual defense bill, which this year outlines $700 billion to improve military readiness, upgrade defense systems and fund combat operations, is an attractive vehicle for controversial measures because it is considered "must-pass" legislation. But McCain is wary of letting lawmakers wage too many policy fights on the bill.

Last week, McCain said he had not read, and "may not even support," proposals to enhance sanctions against North Korea. He was similarly skeptical about Sens. Kirsten Gillibrand (D-N.Y.) and Susan Collins (R-Maine) trying to counter Trump's ban on transgender soldiers, saying that "right now it's not necessary" because Defense Secretary Jim Mattis froze the directive until Feb. 1.

McCain also appears to be ignoring Sen. Rand Paul's attempt to delay progress on the defense bill until Paul (R-Ky.) secures a vote on an amendment phasing out existing AUMFs after six months.

"Why waste my time?" McCain said when asked whether he was speaking with Paul.

The defense bill is, by sheer size, Congress's biggest policy and funding lift in any given year, apart from periodic budget and debt limit fights. For the past two years as chairman of his committee and several years before as the ranking Republican, McCain has refereed policy fights, faced down veto threats and sent the defense bill to the president's desk, where it has ultimately been signed into law for the past 55 years.

There is poignancy in that task this year since McCain announced this summer that he had been diagnosed with brain cancer. His Senate colleagues, Democratic and Republican, pledged to move procedural heaven and earth to ensure McCain could see the defense bill through.

A controversial amendment could complicate that task. Still, McCain's skepticism toward amendments challenging Trump does not mean his defense bill is in lockstep with the president.

On Friday, the White House's Office of Management and Budget released a list of complaints about the Senate's defense bill, pushing back against prohibitions on base closures, restrictions on alternative pay structures for service members and restructuring at the Pentagon. The administration also objected to policy statements in the bill as Congress intruding on the president's constitutional right to conduct diplomacy and be commander in chief.

But the administration stopped short of a veto threat, even praising the bill for helping counter the Islamic State and end defense budget cuts known as sequestration.

That means the main hurdles for the defense bill are policy fights senators might raise.

If McCain is able to get around Paul's threats, the AUMF issue likely will not rise again. The chief agitators for an AUMF, Sens. Tim Kaine (D-Va.) and Jeff Flake (R-Ariz.), are committed to addressing the topic in the Senate Foreign Relations Committee, where they have been promised a markup soon.

Senate Banking Committee Chairman Mike Crapo (R-Idaho) has promised markups, probably next month, to the authors of the North Korea sanctions bills.

But that isn't fast enough for Sen. Cory Gardner (R-Colo.), whose bill blocks firms and banks that deal with North Korea from the U.S. financial system, or for Sens. Chris Van Hollen (D-Md.) and Patrick J. Toomey (R-Pa.), whose bill models secondary sanctions against Pyongyang on the Iran sanctions Congress passed in 2010 and 2012. Their proposal also lets Congress block the president from rolling back sanctions on North Korea; the Trump administration objected vehemently to a similar provision concerning sanctions against Russia that Congress approved in July.

The North Korea proposals' authors want their measures attached to the defense bill. But it's not just McCain resisting their efforts — other leading Republicans also believe it is smarter, right now, to stand down.

"We're much better off trying to work in conjunction, at this juncture, with the White House, instead of jumping out there and trying to pursue something independently," said Senate Foreign Relations Committee Chairman Bob Corker (R-Tenn.). He said the North Korean nuclear crisis has become too "acute" for Congress to act alone.

Gillibrand and Collins may also face difficulty drumming up support for a transgender troops amendment because despite bipartisan backlash to the president's ban, the order has not been implemented. As of the end of last week, the senators' drafts were not ready to circulate among colleagues.

But the main hurdle to proposals challenging Trump may simply be that congressional leaders do not have much time or space to maneuver. The Senate's defense bill must be reconciled with the House's product — a process promising policy fights of its own. And both McCain and his House counterpart are determined not to leave the military underfunded at a time when the president is grappling with so many international crises.

"The most important thing we can do is get our work done . . . and that's a defense budget for the year," House Armed Services Committee Chairman Mac Thornberry (R-Tex.) said. "The most important thing we can do is be strong militarily — that enhances Secretary [Rex] Tillerson's plans in diplomacy, that gets the Chinese's attention. . . that's the most important thing we can do."

Read more at PowerPost