State Sen. Malcolm Smith and city Councilman Dan Halloran were arrested this morning on charges they were plotting to rig this year’s mayoral election through fraud and bribes. The pols allegedly formed an alliance built on cash payments and fraud to get Smith — one of the state’s top Democrats — placed on the GOP mayoral ballot, sources said. FBI agents arrested them both at their Queens homes shortly after 6 a.m. [snip] Prosecutors are planning to lay out the whole sordid scheme during a news conference later today.