The back-and-forth of the general election campaign season seems to have started Monday for Democratic Sen. Elizabeth Warren and Republican challenger Geoff Diehl. Warren, seeking election to a second term on Nov. 6, appeared on WGBH's "Boston Public Radio" where co-host Jim Braude picked up a question tweeted at the show by Diehl's campaign regarding specifics on how much Warren wants to see tax rates increase.