Benjamin Zimmer in his very good blog, Language Log:
Chief Justice John Roberts' administration of the presidential oath to Barack Obama was far from smooth. Early reports differ in saying who stumbled: NBC and ABC say the flub was Roberts', while the AP says it was Obama's. I think both men were a bit nervous, and the error that emerged from their momentary disfluency came down to a problem of adverbial placement.
The Constitution gives the oath as:
“I do solemnly swear (or affirm) that I will faithfully execute the office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States.”