Do the airlines let you take ball in a backpack or duffle bag as carry on? Leaving for nationals soon and trying to figure out if I can get away with just one checked bag. TIA.

I know people that have done it.  don't recall the airline and it was around '98 or '99.  no idea if rules have changed or not.

My friend put his favorite ball, shoes, bowling towels, bowling tool bag in a carry on just in case the checked bag with the rest of his gear was lost.

Call your airline  and ask them about this

It's all up to TSA and how they feel that day. Got burned 2 years ago trying to take a double bag as a carry-on when others did it with no problem. I had even called the airline and thoroughly checked the TSA website.

I always take my Spare ball in my 2 ball tote with my shoes and switch grip thumb holes as my carry on.  One year they gave a hard time about my abralon pads and the Cab to hold the pads. got a 10 minute Q & A about the cab and pads The Rocket scientist at the TSA could not understand why I needed those items to bowl.

other then that time never had a issue with a spare ball I ship all my strike balls just easier for me

just my .02

Been years since I flew with a tote, but I doubt much has changed...I got out of Austin with no issues but had to check it on the return flight out of Newark.  TSA has the final say on what gets into the secure area...it isn't the airline's call. 

That is 100% up to the discretion of the TSA agents.
I've taken a ball in my backpack before, I've also had it denied in the same bag.  Its a risk that I am not willing to take again.