Who said that was a demotion? Lots of governors run for Congress.
There are governors and there are governors.
Actually, there have even been ex-presidents in Congress. John Quincy Adams served in the House after the White House. Andrew Johnson served in the US Senate, after that very body had unfortunately failed by one vote to remove him from the White House.
And then, of course, there was the enormous William Howard Taft, who was appointed Chief Justice in 1921 by Harding, after having been president from 1909-1913.