Except that...she can't. Not in any universe.
The GOP is going to lean in hard on her failure to secure the border (recall that this was her One Job in the administration).

Except it wasn't. Her job was to address the root causes of out-migration from the Central Triangle. Not to secure the border, not to set border policy, not to oversee Customs and Border Patrol, and not to deal with any of the issues resulting from border crossings. At no point was she tasked with reviewing the specific aspects of border security that the GOP complains about.

Regardless of that fact, I completely agree that she's not going to be able to convince even middle-information voters of that. While that wasn't her One Job, the GOP is probably going to be able to make that stick on her. This is, again, one of the downsides to switching candidates in midstream - the GOP is going to get a free and clear two weeks or so to try to control the narrative on Harris, while she's in the process of spinning up her campaign.

But "law and order" isn't just about the border. It's also about crime. That was the triad that Trump was going to use against Biden - inflation, border, crime. Harris won't be running away from her time as a prosecutor - she'll be wielding it proudly as a shield against the "soft on crime"/"defund the police" attacks. Again, those attacks will leave a mark, but she's got countermeasures to deploy that Biden did not.