Incumbent means someone who currently holds office. Is there a word that describes someone who is completely new to politics, like "rookie" does for sports. I feel like "rookie" doesn't fit well with political office, though.
In American English "freshman" is common, especially when discussing legislators.
freshman: lacking seniority or experience; junior:
n. a young or inexperienced person.
adj. new and untried or inexperienced.
Lamont said yesterday: "He is a fledgling politician and a fledgling journalist and he will have learn you do not damage people's reputations that have been hard won.
: a person who has recently started a particular activity.
neophyte : a person who has just started learning or doing something; : a person who has recently joined a religious group
The "religious" group being other politicians.
someone who does not have much skill in what they do
"I won't be giving them any more work - they're a bunch of amateurs."
a person who has just started a job or activity and has little experience
Ngram shows that "amateur politician" was used more than "rookie politician" and it peaked in usage in the 1940s.
Btw, if the politician shows good potential, he may be referred to as a "rising politician".
increasing in rank, status, or reputation: "a rising young politician."