Eliminate the unit rule whereby the winner of the popular vote gets ALL of the state’s electoral votes.
The problem with this is that a small state which does it makes itself worth less to a presidential candidate. It also renders "swing states" impotent because candidates will know that they can always win a number of electoral votes from these states, instead of having to campaign hard to win all of the electoral votes from them.
Maine and Nebraska award one electoral vote each by congressional district, with two electoral votes in each state going to the statewide winner. The other 48 states award them winner take all. Each state could opt to split their electoral votes based on the popular vote within their state, or by congressional district, or some other method.
The only such "unit rule" consists of 48 individual state laws which specify a winner-take-all awarding of the states' electoral votes.
Both Maine and Nebraska award their electoral votes based on congressional district voting. The other 48 states, if they choose to do so, are perfectly free to do that as well.