Giving more money to the very poor raises the prices of the goods that the lower middle class buys.

I don't think most people think about this sort of economics.

They don't want to raise taxes on the rich because they are hoping to be rich themselves in the future.

Yes, this is absolutely part of it - but not in the practical sense. Here's my impression. They don't literally believe (with any confidence) that they're going to be wealthy and high-status in some years or decades, but their belief-in-belief that they do expect it (and that it's a practical, reasoned expectation) is important, identity-building self-signaling to them - and signaling to their lower-middle-class peers too, particularly to "get one up" on the ones who don't engage in this signaling.

It is much (although not quite) like Steinbeck's famous saying: "Socialism never took root in America because the poor see themselves not as an exploited proletariat but as temporarily embarrassed millionaires." Also, as I've noticed, a lot of Western fiction - not exclusively left-wing, even - that touches on the struggles of the lower middle class, the higher-status poor, and the problems with the "American dream" explores this very kind of psychology.

Unlike the Shades, though, Cockbill Street was clean, with the haunting, empty cleanliness you get when people can't afford to waste dirt. For Cockbill Street was where people lived who were worse than poor, because they didn't know how poor they were. If you asked them they would probably say something like 'mustn't grumble' or 'there's far worse off than us' or 'we've always kept uz heads above water and we don't owe nobody nowt'.

He could hear his granny speaking. 'No one's too poor to buy soap.' Of course, many people were. But in Cockbill Street they bought soap just the same. The table might not have any food on it but, by gods, it was well scrubbed. That was Cockbill Street, where what you mainly ate was your pride.

What a mess the world was in, Vimes reflected. Constable Visit had told him the meek would inherit it, and what had the poor devils done to deserve that ?

Cockbill Street people would stand aside to let the meek through. For what kept them in Cockbill Street, mentally and physically, was their vague comprehension that there were rules. And they went through life filled with a quiet, distracted dread that they weren't quite obeying them.

People said that there was one law for the rich and one law for the poor, but it wasn't true. There was no law for those who made the law, and no law for the incorrigibly lawless. All the laws and rules were for those people stupid enough to think like Cockbill Street people.

Terry Pratchett, Feet of Clay

Giving more money to the very poor raises the prices of the goods that the lower middle class buys.

This depends on how much those goods benefit from economies of scale.