By Laura Moser

This week marks the closest Congress has come to replacing the widely reviled No Child Left Behind Act since it officially expired in 2007.

Democrats and Republicans are squabbling over the usual issues: high-stakes testing and whether kids should be able to opt out of it; how accountable teachers and schools should be to those test results; and, more broadly, how much control the federal government should have over education (consensus: not nearly as much).

But one of the most divisive issues under consideration, what’s known as Title I portability, or “financial backpacks,” is also one of the most abstruse. There’s a portability provision in the House bill that passed, barely, Wednesday night, but the subject has become such a flashpoint for Democrats that President Obama has threatened to veto any bill that includes it.

The final bill almost certainly won’t, since the more moderate Senate bill still being debated has omitted any mention of portability, but it’s still worth understanding what exactly portability is and why it’s such a big deal.

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