There have been rampant reports that Johanns wants to run for the Senate seat that is being vacated at the end of next year by Sen. Chuck Hagel, a Republican. Johanns' spokeswoman, Terri Teuber, would say only that the secretary had an announcement planned about his future.
The president's announcement was scheduled for 1330GMT. He also planned a news conference at 1445GMT
A Johanns entry into the Senate sweepstakes would be welcomed by Republicans who lost control of the chamber to the Democrats in the 2006 elections and who also are seeing some key incumbents like Hagel and Sen. John Warner opt against running for re-election.
Bush earlier this week proposed filling one Cabinet vacancy - the position of attorney general - with retired federal judge Michael Mukasey, who was nominated to succeed Alberto Gonzales.