Belts do not stretch, actually they shrink during use.
They wear thinner so sit deeper in the V grooves and get slack.
So a sudden slackening in the belt means that some thing is wrong.
The fixed clutch pulley should be in a slot, check it has not become loose, or for that matter fallen right off.
Check that the belt is actually runing in all of the pulley grooves and not on a shaft.
Most oft seen at the rear.
Go to the Sears web site and print out the parts drawing for the transmission and make sure all the bits are there.
Unless you recently changed the belt for the wrong sized one, you have a problem.
You can not check the pulleys by looking at them running, you have to remove the belt and check each pulley.
I like to hoist the mower up by the front axels till it is standing on its butt so I can get to everything easily.
If you do the same, drain the fuel tank first & remove the battery.
Being a Craftsman it can have any one of a dozen transmission lay outs depending upon what actually made it so If you want specific help either post a lot of photos or better still post the model numbers