Congress is expected to hear another damning testimony against Donald Trump on Tuesday as House Democrats continue their impeachment probe. Alexander Vindman, a veteran and lieutenant colonel in the Army and the top Ukraine expert on Trump's National Security Council, is expected to tell Congress that he was so troubled by the president's foreign policy with Ukraine, and his pressuring Ukrainian president Volodymyr Zelensky to investigate former vice president Joe Biden, that Vindman twice filed internal complaints. In a statement obtained by the New York Times, Vindman wrote, "I did not think it was proper to demand that a foreign government investigate a U.S. citizen, and I was worried about the implications for the U.S. government’s support of Ukraine."

Vindman is the first White House official to cooperate with Congress's impeachment inquiry, which makes his testimony all the more damaging. So Trump surrogates rushed to discredit him before he even appeared before the House Intelligence, Foreign Affairs, and Oversight and Reform committees. Their main tactic so far has been to accuse the Army veteran—who has a Purple Heart for an IED injury in Iraq and served multiple overseas tours—of secretly working for Ukraine.

On Monday night's episode of Laura Ingraham's Fox News show, former deputy attorney general John Yoo breezily suggested that Vindman could be guilty of "espionage" and might be a double agent. Over on CNN, former Wisconsin congressman Sean Duffy said of Vindman, who was born in Ukraine, "I don't know that he's concerned about American policy." He added, "We all have an affinity to our homeland where we came from."

While it seems like an absurd claim, attacking veterans has become a common tactic for GOP operatives. Veterans and their families make useful props at the State of the Union, but they're not off limits to attack if there are political points to score from it.

During his presidential campaign, Trump frequently denigrated veterans and their families, like Gold Star parents Khizr and Ghazala Khan, parents of Humayun Khan, 27-year-old Army captain who died in a car bombing in 2004. They spoke at the 2016 Democratic National Convention in 2016, responding to Trump's repeated promises to ban Muslims from entering the U.S. Khizr accused Trump of never sacrificing anything in his entire life, and he asked, rhetorically, if Trump had ever read the Constitution. Trump replied by claiming he's made "a lot of sacrifices." For evidence, he said, "I’ve created thousands and thousands of jobs," which is indicative of running a business and not much else. The then-presidential nominee also suggested that Ghazala Khan was silent during her husband's speech because "she wasn’t allowed to have anything to say."

Trump also insulted the late senator John McCain, a frequent Trump critic who was a prisoner of war in Vietnam. During his campaign, Trump said, "He’s not a war hero. He was a war hero because he was captured. I like people who weren’t captured."

Also in 2016, then-congresswoman Tammy Duckworth, a U.S. army veteran who lost both of her legs in Iraq, was running to represent Illinois in the U.S. Senate. In March of that year, the National Republican Senatorial Committee tweeted, "Tammy Duckworth has a sad record of not standing up for our veterans." And during a debate against Republican incumbent Mark Kirk, Duckworth, whose mother is Thai and father is American, touted her military bona fides, saying, "My family has served this nation in uniform, going back to the Revolution." Kirkman mocked her for it, saying, "I had forgotten that your parents came all the way from Thailand to serve George Washington." He backtracked later on Twitter, writing, "Sincere apologies to an American hero, Tammy Duckworth." Kirkman lost reelection in November.