That’s exactly what they did in 2021. Biden appointed a Republican, Merrick Garland, as his attorney general. And Garland sat on his ass for two years without bringing any charges. Only when they were shamed into it by House committee investigations did they actually start to move. By then, it was too late. Trump ran out the clock. They prosecuted Trump to appease the base, but they did so late enough that Trump stood no chance of conviction before the election.