First, I wouldn’t assume that there isn't anyone planning to assassinate Harris.

Second, let's not pretend that there hasn't been political violence coming from Trump's side of the aisle. For example, does the phrase "where's Nancy" ring a bell?

Third, Trump has positioned himself as an authoritarian strong man who will not be restrained by laws, facts, institutions or norms. He has embraced extremism and excused violence. He promotes hatred, division and conspiracy theories. He attacks law enforcement and courts when they apply the law to him, while simultaneously threatening to wield them as a political tool to persecute his enemies. He has managed to get the Supreme Court to declare the president to be above the law up to and including being able to order the assassination of political rivals. He has rejected democracy, attempted to steal the last election, and incited a riot at at the capital in order to coerce congress into going along with his coup attempt. When that failed, he called for the constitution to be thrown out, and stated that members of his own administration that didn't support him should be executed. He's wielded his influence in the Republican party to undermine the defense of Ukraine and support for NATO, and has single-handedly weakened NATO by threatening to withdraw and sowing doubt as to whether the US can be counted on to honor the defensive pact at the heart of the alliance. He encouraged opposition to basic public health protections during a pandemic simply because they looked bad politically, fanning the flames of an anti-science and anti-health movement that significantly increased the death toll. He has promised to be a dictator, and wants to purge the federal government of anyone who is not loyal to him and his agenda. He has a base full of rabid nutcases who routinely threaten violence and intimidate those that get in the way or try to enforce the law. He uses Nazi rhetoric and is supported by neo nazis, white supremacists, and other militant groups. He also has the backing of numerous billionaires that wield disgusting amounts of power and influence, and he's made no secret that he is willing to sell policy in exchange for contributions.

And that's not even an exhaustive list, just the first things that come to mind. Quite frankly, Trump is so toxic and such a clear threat to basically everything that America and liberal democracy is supposed to be about that it's not a surprise that someone would take a shot at him.