I'd argue the opposite - they are likely to be one, especially if they've run a successful campaign.
Candidates and parties will canvass the electorate to identify who is (likely) to vote for who so they can put resources in to the right things and make sure likely supporters turn out to vote etc.
That's not to mention that any (prior) correspondence with the politician (or their office) will almost certainly also contain personal data.
OK, that would make sense. That's more likely a party thing where I live (so you wouldn't contact a random MP but someone in an administrative position in the party), but I imagine it's not necessarily the same thing elsewhere.
Candidates and parties will canvass the electorate to identify who is (likely) to vote for who so they can put resources in to the right things and make sure likely supporters turn out to vote etc.
That's not to mention that any (prior) correspondence with the politician (or their office) will almost certainly also contain personal data.