That's the suggestion from the Politico:
Fred Thompson plans to announce Tuesday that his committee to test the waters for a Republican presidential campaign raised slightly more than $3 million in June, substantially less than some backers had hoped, according to Republican sources.
Thompson plans to make the disclosure in a filing with the Internal Revenue Service, as he continues to operate his prospective campaign as a political organization that does not require disclosure to the Federal Election Commission.
Three million doesn't sound terrible, but if you pro-rate the second quarter take for each of the Republican candidates for a per-month average, then Thompson finishes fourth -- behind Giuliani, Romney, and McCain. That seems fine for a non-candidate, but probably not impressive enough to keep the bloom on the Thompson 'campaign.'
Combined with the negative stories about campaign staff reshuffling, it probably indicates that this is a good time to formally announce.