I mean that's essentially what China is doing in allying with Russia and Iran. It's what the early communists in China did in conditionally allying with the KMT against imperialist invaders (but they eventually had to confront them and defeat them).

It can easily get silly though when we're talking about communists participating in bourgeois elections by allying with bourgeoisie parties. Although a case like Brazilian comrades trying to keep Lula over a Bolsonaro may be in its way a move to keep the worst of imperialist comprador control out, a case like Yankee communists voting for democrats can quickly get absurd, where it's essentially just communists demanding nothing for their vote and falling to blackmail that "it will get worse if they don't."

In other words, I'd say a rough litmus test is that there is some kind of strategic gain from it, especially as longer-term strategy. Hell, write a basic pros and cons list if it crystallizes things better. Get it out of the abstract and into what the real consequences are.