I would say that they are harping on about something (for whatever motive).

If you say that someone harps on a subject, or harps on about it, you mean that they keep on talking about it in a way that other people find annoying.

Collins

Trump began promoting Moore by harping on how bad it would be to have a Democratic senator in Alabama.

Axios

The Ngram shows that there is equivalence between AmE usage and BrE.

I would say that they are harping on about something (for whatever motive).

If you say that someone harps on a subject, or harps on about it, you mean that they keep on talking about it in a way that other people find annoying.

Collins

Trump began promoting Moore by harping on how bad it would be to have a Democratic senator in Alabama.

Axios 12th December

The Ngram shows that there is equivalence between AmE usage and BrE.

I would say that they are harping on about something (for whatever motive).

If you say that someone harps on a subject, or harps on about it, you mean that they keep on talking about it in a way that other people find annoying.

Collins

Trump began promoting Moore by harping on how bad it would be to have a Democratic senator in Alabama.

[Axios][2] [2]:https://www.axios.com/3-how-trump-came-to-love-roy-moore-2516690256.html

I would say that they are harping on about something (for whatever motive).

If you say that someone harps on a subject, or harps on about it, you mean that they keep on talking about it in a way that other people find annoying.

Collins

Trump began promoting Moore by harping on how bad it would be to have a Democratic senator in Alabama.

Axios

The Ngram shows that there is equivalence between AmE usage and BrE.