The Senate will vote on a procedural measure to advance the National Defense Authorization Act’s conference report Tuesday, under a pending veto threat by President Obama that complicates the road ahead.

The Senate passed its version of the NDAA earlier this year by a vote of 71 to 25 – plenty of support to clear both a 60-vote filibuster threshold and even override Obama’s promised veto.

But the president has since threatened to veto the conference report over its use of extra Overseas Contingency Operations (OCO) funds – money that essentially serves as a workaround to the sequestration caps in place across the budget.

The OCO funding maneuver existed in the version of the bill the Senate supported earlier. But it’s not clear whether the renewed threat will deter Democrats from supporting a procedural motion to advance the bill.

Only two Senate Democrats on the conference committee – Sens. Tim Kaine (D-Va.) and Joe Donnelly (D-Ind.) — signed off on the conference report. All of the others, including Armed Services Committee Ranking Member Jack Reed (R.I.), refused, citing concerns with the OCO funding mechanism.

Yet those Democrats opposed the Senate’s version of the NDAA in June as well. And Republicans are speaking as if veto-proof support for the bill is expected again.

“Americans have every reason to expect that Democrats will vote again to support – not block – America’s national defense authorization bill,” Senate Majority Leader Mitch McConnell (R-Ky.) said Tuesday, charging that Obama was “goading” Democrats into opposing the legislation, just as the military needed all the tools it can get to fight growing global threats.

Opponents of the NDAA’s conference report argue that the OCO funding is a short-term strategy at best, and will only incentivize Republicans who want to avoid busting the sequestration caps budget to hold off talking about that.

But Republicans – and some Democrats who support the conference report – argue that settling the OCO issue is a matter for appropriators, not an authorization bill.

The House passed the NDAA conference report last week by a vote of 269 to 151 – that’s a margin too small to overcome the president’s veto.