The Trump program of deterrence through unlawful cruelty inevitably led to a backlash that shaped the 2020 election. Harris was caught up in it; like all of the Democratic candidates other than Biden, she took positions on the border during the debates that she undoubtedly regrets today. When she became VP, however, she followed the Biden line without public dissent.
Biden wanted a system that was humane and generous, but orderly. In spite of his best efforts, circumstances overwhelmed him: the end of the pandemic took away his best reason to keep migrants in Mexico; political turbulence and climate change all over the world made immigration more attractive; left-wing legal assaults on his program were largely successful; the system was underfunded; and the GOP was more interested in scoring political points than in moving legislation to improve the system.
Today, the border is the biggest issue in GOP commercials all over the country. Harris is fighting back by touting her past as a prosecutor and by blaming Trump for the failure of the border legislation. Will that work, and does her new hard line represent her true position on the issue? As to the first question, the answer is no, but she only needs to blunt the attacks, not to make them disappear; as to the second, Harris doesn’t appear to have strong convictions on immigration, so don’t expect a sudden shift to open borders if she wins unless the wind starts blowing hard from the left, which is unlikely.