I graduated with a Ph.D. in a STEM field from a good university in the US a few months ago. Thinking that I will benefit from a diverse experience, I did not continue under my professor for a postdoc position. I was jobless for the last few months, since much fewer job openings are currently available in my field. To add to the troubles, my employment authorization card (I am an international student) arrived very late due to the pandemic situation, which rendered me helpless. Now that reality has dawned on me, I was thinking of reaching out to my professor for support for the next few months while I keep searching for a job. A disclosure: my performance while working under him wasn't up to his expectation. But, he is a very nice guy and may show some mercy on me. Call it anxiety or fear of rejection, I have not taken any steps towards that yet. I Will greatly appreciate some insights on how I should approach this situation.
I think that reaching out and asking never hurts. The worst case is that they say no. Just explain your current situation, and why you're reaching out so late. Do expect a rejection, not because of any issues they may have had with you in the past, but rather because postdocs are not cheap, and taking on an unplanned group member is never easy to do. This has nothing to do with your performance, just that hiring someone requires a budget and time to work with them.
If they agree to interview you and discuss your options, explain your plan for how to make things work this time around: alleviate any concerns they might have about hiring you. Treat this like you would any other interview, and come prepared.
You should not expect them to take you out of the kindness of their heart.
Show them how you can make their research better.
If you do an impressive enough job at that interview, and they have no opening, they may still offer you options that do not require them to hire you. For example, there may be teaching positions in the university or relevant industry jobs that they may be aware of and whose profile you fit.