This time I was going to be prepared. The leak was from the valve stem. Tightening down the nut didn't work so I either had to 1) replace the stem, 2) replace the valve, or 3) pack the stem with with graphite packing string to stop the leak. Option 3 was the easiest and probably.
I went to the hardware store hoping they would have graphite packing string. I didn't trust Home Depot to carry it. My list also included a supply line- which I should have bought the other day- and a replacement valve. I didn't want to go back to the store for parts.
I shut the water off and undid the nut. I pulled the stem out and took a look at it. It was plastic and in good shape. The packing washer though appeared to have shrunken up. Hence the leak. I strung the packing string around the stem and tried to get it back in place. Easier said than done.
I realized I had too much of the packing string around the stem and began to remove some. Soon I had it just right and it was all put back together. I ran down the two flights of stairs and turned the water back on. Back up the stairs and opened the valve.
Yes, the stem isn't leaking any more! But there is a leak. WTF?
I took a close look and noticed it was leaking where the supply line connected to the valve. Wait. Did I miss this? Was it leaking at this connection all the time and not the stem? Did I misdiagnose the problem?
I replaced the supply line and turned the water back on. No leak. It was done. This took way more time than it should have. But it is done and I am content.
Now I have the rest of the day to drink. Yea me!