I suspect it will not be which "legacy" issue the Dems will undo, but rather the most critical policies they will revert back to prior to his election to do harm U.S patriotism and future prospects.
In my opinion this revolves primarily around his illegal immigration policy which in and of itself could save hundreds of thousands of lives (assuming many of the illegals being deported are poison dealers or serious criminals) over the period of four years in his term. This is easily the most important policy he has applied with ICE.
To quote the CCP, who regardless of what ones opinion might be of the Communist system are very astute in regards to their own MCGA policies, when suggesting that their population could decrease by hundreds of millions, something to the effect, "we do not care about the numbers, we just want the right kind of citizen". Meaning, it is better to accept a decrease, it will not impact their GDP per capita and might in fact increase it. However, they will be loyal Chinese, hard working, motivated etc. That is their plan at least.
How does this compare to Western Europe or some in America who want to accept low skill labour, something that will be rapidly replaced by A.I anyways and often from people who cannot even speak English and have entirely, incompatible belief systems?.
To me this is the most critical policy to Americas success. Canada has destroyed some of our most ambitious. I am trying to be objective when I say that of all the people I knew I was the most headstrong and driven, all for nought in this country. This seems to be the self destructive cowardice emulated in other Western European nations who will be beholden to their new Chinese overlords (or Russian) in due time at this rate.
If the Dems re-open their borders, which many of the global socialists want as they wish China to replace you and Judeo-Christian values; they will in fact succeed in this evil endeavour.
Whoever wins in 2028 must run on a promise to protect your borders and focus on AmerIcan prosperity and innovation, not low hanging fruit. You can select a few hard working people on faith alone, but the world has changed, it isn't the 1920s and requirements of all successful, future focused nations must adapt.