LAS VEGAS — For weeks, Republicans have pummeled Vice President Kamala Harris on immigration, blaming her for President Joe Biden’s policies at the border. Now, Harris, the Democratic presidential nominee, is seeking to neutralize that line of attack, one of her biggest weaknesses with voters, running a playbook that Democrats say has worked for them in recent elections and staking out her clearest position yet as a tough-on-crime prosecutor focused on securing the border. This week, she has hit back by promising to heighten border security if elected and slamming her Republican opponent, former President Donald Trump, for helping kill...