Quoted from Could Britain have a black PM? (BBC news)
An MP needs to get within electable distance of the leadership of their party and that usually means a successful stint in cabinet or shadow cabinet.
It means to get into a position where it might be possible for him to be elected to the top job. One way to do this (the usual way, according to the writer) is to serve successfully in the cabinet or in the "shadow" cabinet. In other words, you can't show up out of nowhere and expect to be chosen as leader; first you must prove yourself in lesser positions, and some positions seem to lead more naturally to the top than others.
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