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Beyond a vehicle for advertising, though, Priorities has made itself into a research operation, spending a quarter-million of the $8 million it spent in June digging up ammunition for use against Mr. Romney, records show.

The pro-Romney group has the upper hand monetarily — Restore Our Future has $22 million in the bank, while Priorities has less than $3 million — and appears to have opted for a lie-in-wait strategy.