Paul Ryan was the top vice-presidential choice of the conservative punditocracy in Washington. He was also the top choice of the Democratic operatives who have long been readying an attack against Mitt Romney's running mate.

When those sides agree, it doesn't mean peace - it means war.

Read More About The Case For and the Case Against Paul Ryan

The choice of Ryan is simultaneously designed to be satisfying to the Republican base and provocative to much of the rest of the country - in short: bold, brash, and possibly explosive.

Ryan's selection will ignite a major ideological battle between the two sides, the very fight the nation has been poised to have since the Tea Party takeover of 2010. Ryan has been perhaps the most articulate messenger for a major rethinking of the federal government's relationship to its citizens, and gives Romney immediate policy heft.

The biggest shift could come in the campaign's overarching themes. A campaign that has at times seemed to be about nothing will now be about something big, or more accurately a series of very big things.

Choosing Ryan is a tacit acknowledgment by the campaign that its initial assumptions about the race - that it's a coin flip, that Romney's biography and experience could speak for itself, that making the race a referendum on President Obama was enough - no longer apply. Yes, Romney had already embraced the Ryan budget. But he'd done so tenderly and reluctantly, checking a box. Now Romney has invited Ryan himself into his own box.

No other choice came with ready-made policy that's anywhere near as defining for the candidate or the party. No other candidate would immediately tip the balance of the campaign like Ryan. The question for the new Romney-Ryan is whether it will tip in their direction, or wildly in the other way.