A BE36 Pilot reported loss of power that was later traced to improper use of mixture control resulting in flooding.

Narrative

Took off on an IFR flight. We climbed to our cruise altitude of 10;000 feet. We were 45 minutes into our flight [when] I felt the engine produce partial power. I immediately switched the fuel tank and turned the fuel pump on. The engine was still only producing partial power so I put the mixture control to full rich. When the engine still did not produce full power; (for that altitude) I quickly loaded the nearest airport into the GPS. I asked ATC for the closest airport already knowing where it was. They responded 'heading 200; 12 miles'. They asked if everything was ok. I told them I was having engine problems and they immediately asked if I would like to 'declare an emergency'; my response was yes. They told me changing to advisory frequency was approved 6 miles from the airport. We landed with no problem and we were able to resolve the issue. The fuel selector must have been between detents. When the engine performed as it did; my first reaction was to switch tanks. I immediately turned on the fuel pump and mixture went to full rich. The engine was now being flooded and would still only produce partial power. We landed with this configuration. After talking with my mechanic I now know it was pilot error.