Barack Obama has always excited big Labor and other Democratic constituencies, even if they weren't necessarily ready to endorse him for the presidency. Now, though, as Mr. Obama goes all out to undermine a surging John Edwards in the final days before the Iowa caucuses, that cordial relationship is being put to the test. Mr. Obama has essentially charged that Mr. Edwards is a hypocrite because, while Mr. Edwards has said he favors banning unregulated campaign money from outside groups, his campaign is getting millions of dollars worth of assistance from outside contributors -- namely unions. What's remarkable about this line of attack is that, while Mr. Obama is aiming squarely at Mr. Edwards, he also risks alienating some of the most powerful interest groups in Democratic Washington.In case you haven't been paying attention, here's what's going on: several state chapters of the mighty Service Employees International Union have endorsed Mr. Edwards and set up an independent entity -- the kind of so-called 527 that Edwards himself has decried in the past -- to run ads on his behalf. They even hired his 2004 campaign manager, Nick Baldick, to oversee the operation. (The state and municipal workers union and Emily's List are also in Iowa, dropping a lot of cash in support of Hillary Clinton.) Mr. Obama, the only one of the three leading candidates who hasn't received a big union endorsement, has repeatedly called on Mr. Edwards to ask his labor friends to stand down. Mr. Edwards has responded by saying that he still opposes such groups in theory, but that he isn't involved in the union effort and is powerless to control it.