Reply: Is the lectern mic a gooseneck type? If so, place it about 8 inches from the mouth. Then you can turn down its volume, which reduces feedback.
Can you just turn down the volume of the lectern mic so that it doesn't feed back, or is it not loud enough then? Can you equalize (turn down) the frequency that is feeding back in the lectern mic using your mixer's equalizers?
You could plug in a 1/3-octave graphic equalizer between your mixer output and your power-amp input. Start with all the graphic EQ controls flat. With no audience present, slowly turn up the lectern mic until it feeds back. Turn down (and then return to "flat") each control on the graphic equalizer one at a time until you find the control that stops the feedback. Once the feedback stops, turn up the lectern mic a little more until it feeds back at a new frequency. Find the control that stops the feedback, and turn it down until the feedback stops.
Some mixers have insert jacks, so you can connect the graphic EQ to the insert jack of the lectern mic channel. That will put the graphic EQ inline with only that mic's signal. The Mackie insert jack, for example, is a stereo phone jack of a tip-ring-sleeve type. The tip terminal is the insert send, the ring terminal is the insert return, and the sleeve terminal is the common cable-shield connection.
You need to make or buy an adapter cable: TRS (stereo) phone plug on the mixer end, and two RCA or 1/4" phone connectors on the graphic EQ end (whatever mates with your graphic EQ's connectors). Wire the TRS tip terminal to the graphic input; wire the TRS ring terminal to the graphic output, and wire the TRS sleeve terminal to the shields of both cables.