Trump does unpopular stuff, and he becomes more unpopular. The erosion mostly comes from independents because Republicans are highly loyal to him and Democrats are already almost uniformly opposed.
But Trump will need those independents to win re-election. He needed them to become president in the first place. Trump won independents by 4 points, 46 percent to 42 percent for Hillary Clinton, in 2016. Had they gone for Clinton by 4 points instead, she would have won the national popular vote by 4 or 5 points, and won Wisconsin, Michigan, Pennsylvania, Florida and possibly Arizona.
Or things could get a lot worse than that for Trump, and he could lose independents by a wider margin. In the 2018 midterms, Republicans lost independents by 12 percentage points, contributing to a 40-seat loss in the House. The key facet about the midterms is that turnout was very high, including among the Republican base. But it was also high among the Democratic base, and Republicans badly lost independents. The base alone isn’t enough to win national elections, especially for Republicans, since fewer voters identify as Republicans than as Democrats.