One poorly conceived critical passage in the letter Sen. Barack Obama (D-Ill.) shot off to Sen. John McCain (R-Ariz.) over ethics and lobby reform helped provoke McCain’s over-the-top slapdown.

Obama told me that McCain “misunderstood” that crucial part of the letter, having to do with the creation of a task force to chew on lobbying and ethics reform proposals, rather than have bills go through the regular committee structure.

From McCain’s perspective, there was no misunderstanding at all, just an unbased accusation.

“The perception in our office was that this was a very innocuous boilerplate letter,” Obama told me the day after McCain accused the freshman senator of “self-interested partisan posturing” and “disingenuousness” in a ferocious response to a letter he received from the freshman senator.

McCain questioned whether Obama was backing away from personal pledges to negotiate a bipartisan deal for tougher ethics and lobbying laws. “I would like to apologize to you for assuming that your private assurances to me regarding your desire to cooperate in our efforts to negotiate bipartisan lobbying reform legislation were sincere,” McCain wrote.

The letter that Obama sent to McCain last Thursday, when he was on a plane to Germany, and that was released to reporters by the office of Senate Democratic Leader Harry Reid (Nev.) on Friday — turned out to be the first volley in war of words.

Obama sent a letter that picked him a fight.

The spat engaged two popular senators (each potentially eyeing the White House), both taking leading roles in ethics and lobbying reform.

By agreeing to this partisan assignment, Obama is stepping out for the first time from the apolitical role he fashioned for himself during his rookie year in office. It’s also the only time anyone laid a glove on him since he started running for the Senate.

For Obama, accustomed to glowing press, it was a big-league mitt. The front page of Tuesday’s Chicago Sun-Times headlined — in eye-chart-popping big letters —  “McCain Mocks Obama.”

While McCain heard nuclear thunder in the Obama letter, Obama thought he merely lobbed a stone in a slingshot.

“I think his feelings got bruised,” Obama told me.

What McCain found especially troubling was a sentence where Obama stated, “I know you have expressed an interest in creating a task force to further study and discuss these matters.” Obama went on to write that it was time to let the committees of jurisdiction “roll up their sleeves and get to work,” which I am told troubled McCain because it made him look like a slacker.

On the central point: McCain never suggested forming a task force. The idea was floated last month from Senate Majority Leader Bill Frist (R-Tenn.). Reid rejected the notion on the grounds it would “only slow the process while providing senators with political cover.”

Obama told me the bottom line is that McCain “misunderstood the letter. I think there was confusion over the reference to a task force. We were specifically referring to the proposal that Bill Frist had had, to set up a formal task force to do this.”

Obama said he thought his letter only “memorialized stuff that John and I already talked about.”

But the lack of clarity in the Obama letter over the task force created a chain of events the so-far cautious Obama never, ever anticipated.

Sweet is the Washington bureau chief for the Chicago Sun-Times. E-mail: This email address is being protected from spam bots, you need Javascript enabled to view it