Cattleya has gone from great to grievous
 

After repotting a new cattleya with a good potting mix (orchiata), it has started losing leaves, and the pseudobulbs are dying and shriveling. The temperature is warm in the summer - upper 70s, low 80s. The other catts, growing in the same indoor space, seem to be doing okay. My gut tells me the orchid is underwatered, with the high temperature and fresh potting mix. What do you think?

Photos would help with the diagnosis. And information about what you found in the way of roots system when you unpotted it. The time to repot Catts is when they start producing new roots. Some are fussier than others about the timing of repotting.

Certainly, new medium will need watering more often. While old roots tend to not adjust to new medium unless it's pretty similar to the old, if they are in decent condition they will maintain the plant until the new roots establish in their new home. Certainly leaf loss is one way that a plant reacts to dehydration, it's trying to minimize water loss. The temperature appears to be pretty moderate - most Catts not only tolerate higher temperatures, but many like it.

But there's too much information missing here to give advice with any accuracy. First, do you have a name for the plant? Then, need one or more photos, detailed notes about condition when it was repotted, any treatments that you performed while you were doing the repotting. So... a lot more detail needed, otherwise everything is speculation.

I too would ask whether it was making new roots when you repotted. Cattleyas may make new roots only once or a few times per year. When repotting roots are damaged. If the plant won't make roots for some time after repotting it may struggle to take up enough water through the old, damaged roots. Species and bifoliate Catts are particularly sensitive about this.

If it wasn't rooting when you repotted your plant is likely to survive but it may be set back for a while.

Photos of Cattleya
 

To answer your questions: No, it was not growing new roots when I potted it. I'm not sure what kind it is, but it is unifoliate.

What's your advice on helping it to recover?

I think just patience. Many of these are really survivors. Where there is green there is hope! I would not try to remove it from the basket if that's what it's in. Just drop the whole thing into whatever pot it will fit, add some large bark, water like the rest of your Catts, and it will root when it is ready. The plant looks quite healthy.

Thank you for giving me hope 🤞🏼

Don't be afraid to water it! If it is in a pot or basket that drains well and in large bark, you really can't overwater, especially this time of year. Roots look OK, so it should be able to utilize what you give it.