It can be tight due to rust as you say. Put the nut back on and thread it nearly all the way home. Put the socket on/over the nut and then hit the socket. The socket will protect the nut and the nut will protect the threaded end of the axle.

Also, with the nut undone, try placing a socket on the other end and turning the axle/shaft as if you are tightening the head of the axle. Sometimes this can work when applying clockwise force for a fraction of a turn ( just a nudge ) to break the grip of the rust.

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