They did impeach him. And if it was illegal, the reps could have done something about it, which they didnt.
If Trump accepts if he loses and then has enough to start investigating because he thinks it was fraudulent, he/they obviously can do that, just like the dems did. Dems felt like an investigation was needed, so they did. If reps feel the same now, they can. But thats different from not accepting the results and not handing over the power to the next president, which the dems did in 2016.