The SJC has ruled that the President is immune from lawsuits related to his official duties. Of course, we may all reasonably claim that trying to overturn an election is hardly an official duty, but the WAPO article says that the exemption has been interpreted broadly.
One could argue that Trump was defending the legitimacy of our democracy by trying to reject fraudulent election results. Even though the claims of fraud have been dismissed by the courts, it may be enough that he genuinely[1] believed that the results were fraudulent and required fixing. The fact that he was mistaken wouldn't mean he wasn't actually doing what he regarded as his duty.
Few people here would take the argument seriously, I'm sure, but I wonder how many justices would buy it.