Are there any idioms to say “secretly support someone or something “?

There is a notion of being 'in the closet'. Typically it refers to keeping the fact that you are gay a secret, but it can apply to other contexts, so: 1. This US citizen is a closet SVR sympathiser. 2. She acts like a Swiftie, but is actually a closet Kanye fan. sound ok to me. \--- There may also be some specific ones. Like "watermelon" is sometimes used as to describe/accuse people of acting as if their agenda is environmentalism, but their actual secret agenda is socialism/communism. The idea here is that they are "green on the outside, red on the inside". There could be more, but since some conservative commentators are a fan of very freely calling things 'socialist', I think this is the only one that comes to my mind.

"Off the record" or "On the QT" or "On the sly" (less common), perhaps?

On the down low Or Low key She’s a swiftie but a lowkey Kanye fan

You could say "closeted" which comes from gay culture like when someone is in the closet means it's a secret or if they're out of the closet means they've told people that they're gay. In the US. This is a generally accepted (but idiomatic) way to say secretly a fan of something. "He says he only listens to country music, but he's a closeted Taylor Swift fan"

Subversive?

Covert, Sympathizer, Clandestine.

I just say secret supporter of so and so

In America we used to call them turncoats which is switching sides turning your coat inside out to hide the insignia patch.... the other term we sometimes use is Benedict arnold to reference the man who switched from the American side to the British side....

You’ll sometimes see the “crypto-” prefix. E.g. a [cryptofascist](https://www.merriam-webster.com/dictionary/crypto-fascist). (This includes a hyphen, but I sometimes see it without.)

You could say "clandestinely" instead of "secretly," it implies political machinations/efforts. You could call someone a "wolf in sheep's clothing" if you also want to imply that the person/faction they secretly support is threatening and the one they pretend to support is sweet/innocuous. You could call someone a Benedict Arnold if the cause they secretly support is politically opposed to the cause they pretend to support. (Ie they're a traitor). Pretty sure that's US-specific as he was a figure during the US Revolutionary War. You could say someone is part of "the [secret cause] Resistance" if you also want to say that the cause they secretly support is just and the cause they pretend to support is tyrannous. But the two causes would need to be very well known to be opposed. You could say their support for the pretend cause/person is "as real as [____Celebrity's] known-to-be-fake [thing]." Eg Kim Kardashian's ass, Elon Musk's hair, Gwyneth Paltrow's health advice, JLo's kindness, Mark Zuckerberg's chain." In that vein you could say they're as trustworthy as a drink from Bill Cosby. Or as safe as a hotel room with Harvey Weinstein. You could say they're a ___ fan "in name only." You could try to call someone a Schindler with a List if you want to say that the cause they secretly support is just and the cause they pretend to support is tyrannous. It's not actually an idiom but I think most people know of the figure via the movie.

Enjoying the variety of good and different answers here. I would say "covertly support" assuming the person's intention is to keep their support secret. ... "Inconspicuously/privately support" if they're not public about it but might discuss it within their circle. For example, a rich anonymous donor to charity. A spy, double agent, or defector might be called (as epithets) a traitor or turncoat. ... Backstabbers may also be described as "double-crossing" or "a snake in the grass." If supporting a cause or business with funds etc. one might be called a silent partner or silent backer.

On the low Low key

He's an American citizen but surreptitiously supports Russian intelligence agencies. Love that word. Literally means done sneakily or under the table.

Not an idiom but clandestinely is a great word

"Double agent" could work too although it might sound a bit cheeky in non-serious/unofficial contexts. But it will still work.

Carries a torch for

*Behind the scenes.*

stan is a word for being a supporter or fan of something. Like "I secretly stan taylor swift"

Where I’m from we would say cahoots, like “he is an American citizen but he’s in cahoots with Russia’s intelligence agencies”.

you might see the idea of a "sleeper agent"., a la "He act's progressive, but he's really a conservative sleeper vote"

Surreptitious works. Something done surreptitiously is kept secret, Especially if it's something that I bring disapproval The detective hid behind The other bus passengers while surreptitiously keeping an eye on the subject at the front of the bus.

on the down low, behind the scenes, in the dark\* and possibly the word turncoat, spy, secret agent, double agent or traitor could work for the first one \*=not sure about these

I think I prefer “on the side” [(3)](https://en.m.wiktionary.org/wiki/on_the_side) for your examples. “He’s an American citizen but he supports Russian intelligence agencies on the side.” “She said she’s a swiftie but it turns out she’s listening to Kanye on the side.”

The most current slang for this is “low key”, or the common one that is going out of style, “on the own low”, so you could say, “she’s a swiftie but she lowkey loves Kanye. Doesn’t quite work well with serious topics like being a spy for Russia though. It’s informal slang.

Well, crypto-fascism / crypto-fascist means someone who supports or wants fascism and keeps it secret. So, could use that prefix for... Crypto-russkie or crypto-yeezy or whatever his fans are called.

…they “down low” support X Also, “sub rosa,” “on the QT.”