PHILADELPHIA — It was billed as a blockbuster, but ended up a flop. If Doug Mastriano’s “big rally” last weekend were a movie, its Rotten Tomatoes score would be in the single digits. Only a few dozen supporters joined the Republican state senator on the steps of the Capitol building in Harrisburg — some of them members of a local militia group. The photos looked bad, because it was bad. The press coverage? Brutal. A few days earlier, Mastriano’s fundraising troubles had become clear. He’d appeared on a conservative podcast and asked if perhaps U.S. citizens living abroad might consider contri…