It's possible that a jammer would work, but there are a couple of things to be concerned about. First, jammers that emit RF above a certain power output are illegal across the US and in most of the world.
Next, there are many types of RFID technology in use, and they are all different. You would need to determine which kind of technology you are concerned about protecting.
With NFC technology, as is used in payment cards (you specifically mentioned credit cards), your jammer idea may seriously backfire on you. NFC works by absorbing the RF energy, and using it to power an on-board CPU. The CPU modulates the receiving antenna by applying an alternating high or low load; this switching back and forth causes bits to be detectable by the receiver. The jammer will provide enough RF energy to activate your card, causing it to continually reflect your payment data even when you're not near a reader. It's also entirely possible that a pocket-sized jammer will not have sufficient power to overload the receiver; it may still pick up the NFC reflection from the card even in the presence of a jamming signal. The reader will not discriminate what RF energy source is powering the CPU on your card, it will still be able to read the signal from the reflection. Your jammer might even give your card extra range.
I don't know the legal threshold for jamming, but intentionally interfering with radio reception is virtually always a bad idea. Given you titled this question "RFID safety defense", a jammer is offensive, not defensive, and it would increase your risk of being arrested and prosecuted, which does not increase your safety. Plus, it likely wouldn't work the way you think it might. For defense, you would be much better off shielding your cards, using legal, proven, and cheap shielding technology.
