Competing groups of senators are laying out arguments in what’s likely to be the biggest defense budget issue facing the next administration and Congress: how much to spend on nuclear arms?

The pro-nuclear camp includes Virginia Sen. Tim Kaine , whom presumptive Democratic nominee Hillary Clinton picked as her running mate.

The groups made their respective cases in dueling letters to the Obama administration, which is said to be weighing both scaling back a planned modernization of the nuclear arsenal, and a set of a revisions to U.S. nuclear policy. Regardless of what President Barack Obama proposes, the next commander in chief and future Congresses will be the ones to ultimately resolve these questions.

At issue is upward of $1 trillion in new spending over the coming decade to replace aging aircraft, missiles, submarines and warheads and to upgrade infrastructure. Also up for discussion are matters such as whether the United States should pledge not to be the first to use nuclear weapons or whether it should take its atomic missiles off hair-trigger alert status.

A draft of the Democratic Party platform approved this week at the Democratic National Convention in Philadelphia advocates controlling spending on nuclear weapons and being less dependent on them, a position that is closer to the anti-nuclear senators’ view than to that of Kaine and his camp.