In other words, by passing a simple bill, Democrats would do what they can to align Republican incentives the way they want. At no cost to themselves.
And it’s the right thing to do from a good-government perspective. It’s always been a mistake to insert members of Congress into the presidential line of succession; it’s contrary to the entire structure of the constitutional system, which separates legislative from executive institutions and forces them to share powers.
It also violates the basic partisan arrangements of U.S. elections. Once the political parties evolved, it became essential for the president and vice president to be political allies to guarantee that voters on the winning side of presidential elections would be getting their way, at least in terms of party, even if the president died or needed to be replaced. That wasn’t guaranteed at first by the Constitution, which simply took the candidates with the top two electoral-vote totals and made them president and vice president, and which also had no procedure for filling a vacancy in the vice presidency.