Last week at the Democratic National Convention, Khizar Khan gave an impassioned speech criticizing Donald Trump for his comments about banning Muslims from entering the United States, and endorsing Hillary Clinton in her campaign for President. Khan and his wife, Ghazala, are the mourning parents of U.S. Army Captain Humayun Khan, who was killed in Iraq in 2004.

Khan and his speech, which railed Trump for not respecting the Constitution and the rights of citizens, turned out to be the surprise star of the Convention. The pocket Constitution that Khan brandished during his speech has become the #1 Best Seller on Amazon this week, and Khan has made multiple media appearances regarding his speech.

There are many ways that Trump could have tactfully responded to Khan and his speech. He could have railed at Clinton for voting for the war that killed Khan's son. He could have acknowledged the sacrifice of Humayun Khan. He could have defended his stance as an issue of policy.

Instead, in typical Trump fashion, he decided to attack the Khans on a personal level.

In an interview with George Stephanopoulos, Trump suggested that Khan barred his wife, Ghazala, from speaking during his speech, despite both of them appearing on stage. Trump told Stephanopoulos:

"I saw him. He was very emotional, and probably looked like a nice guy to me. His wife, if you look at his wife she was standing there, she had nothing to say. Probably, maybe she wasn't allowed to have anything to say, you tell me, but plenty of people have written that. She was extremely quiet and it looked like she had nothing to say, a lot of people have said that, and personally I watched him, I wish him the best of luck."

This type of attack is typical of Trump. He has attacked many other people on personal levels in the past, such as attacking Heidi Cruz's appearance, suggesting that Megyn Kelly was impaired in judgement by her menstrual cycle, comparing Ben Carson to a pedophile, et cetera.

However, his attacks on the Khan family are not only personal and disparaging, they also alienate a key voting block that Trump needs to win in November: military personnel.

According to Patchwork Nation, members of the military typically vote Republican. In 2000, George W. Bush beat Al Gore 54% to 44% of the military vote. He beat John Kerry 56% to 43% in 2004. In recent years this gap has closed, with John McCain only leading Barack Obama 51% - 48% in 2008, and Romney only leading Obama 51% - 47%. It appears that military voters, which make up 10% of the nation's potential voters, are a key constituency for Republicans to win. When the margin of victory among military personnel has exceeded 5 points in the last 16 years, Republicans have won the White House. Of course, this is not the only group of voters that decides the election. The point here is that military voters are a demographic that reliably vote for Republicans in the Presidential election.

By attacking the Khan family, Trump has alienated military families and voters even further than he already has with his attacks on John McCain. The Khans have doubled down on their attacks on Trump, and cited the fact that John McCain was their son's hero. The fact of the matter is, attacking someone who dedicated himself to the United States military, and paid the ultimate sacrifice, is a move that would already be unpopular. Attacking his family is political suicide.

The Khans endorsing Hillary Clinton may not have done much to sway military families to vote Democrat this fall, as Clinton is already widely unpopular in many military circles. But Trump's attacks on the Khans will almost certainly sway some military voters away from him. Recent polls show a plurality of active military members supporting Libertarian candidate Gary Johnson over both Trump and Clinton.

Trump will still probably do better than Clinton among military personnel. But how much better than her will he do? Without winning this key constituency of voters that typically vote Republican, Trump has no chance at winning the general election.