"We're doing this now because the politics we believe in does not start with expensive TV ads or extravaganzas, but with you - with people organizing block-by-block, talking to neighbors, co-workers, and friends. And that kind of campaign takes time to build," Obama said in the email.
The president plans to file the necessary papers with the Federal Election Commission as early as this week.
Aides said there won't be any major changes to the president's routine until Republicans settle on a nominee.
So far, no Republicans have formally announced a presidential bid.
The Associated Press contributed to this report.