Last night, the Democratic National Convention found a theme to counter the GOP's "We Built It" night: Government matters, too.

In a low-key but on-point address, San Antonio Mayor Julian Castro lent support to the president's infamous "you didn't build that" bungle, pointing out that our roads, bridges, and schools represent a bedrock of necessary government spending that helps everyone, from thriving entrepreneurs to lower-income families that need a way to get from here to there.

"We all understand that freedom isn't free," Castro said. "What Romney and Ryan don't understand is that neither is opportunity. We have to invest in it."

We can debate how much and what kind of government spending supports or blocks individual opportunity -- in fact, that might be the central debate of this election -- but what was striking to me about Castro's speech and the Deval Patrick address was how Democrats made a clear argument but stopped at its conclusion: If government is so necessary and critical today, where is the argument for more of it?

Stimulus is the word Democrats dare not speak. Treasury yields are still historically low. It will never be cheaper to borrow money to pay for mandatory future expenses to infrastructure. But rather than make the case for short-term stimulus -- as the president himself did in his failed American Jobs Act -- the emphasis has shifted to fiscal responsibility and the contrast between Obama's plan to raise taxes and slow growth in government and Romney's plan to dramatically cut taxes and cut deeply into future discretionary spending.

Obama is still polling slightly ahead of Romney, but Republicans have already won a broader battle over stimulus, a dirty word that has been so demonized that Democrats appear terrified of saying it, even though it was Obama's first accomplishment, his last major budget proposal, and the only reasonable conclusion to the argument that America has unmet needs today that only government spending can solve.