Former CIA chief John Brennan John Owen BrennanJournalism or partisanship? The media's mistakes of 2016 continue in 2020 Comey on Clinton tweet: 'I regret only being involved in the 2016 election' Ex-CIA Director Brennan questioned for 8 hours in Durham review of Russia probe MORE fired back on Tuesday after President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE questioned why witnesses in the House's impeachment inquiry would have been "concerned" with his phone call with Ukraine's president.

In a tweet, Brennan urged U.S. intelligence officials to remain "resolute" in the face of criticism from the president, who has spent the past several days sharply criticizing a whistleblower and others who raised concerns about his conversations with Ukraine's Volodymyr Zelensky about opening investigations targeting Democrats.

"As in previous times of National peril, we rely on our military, diplomats, intelligence officials, law enforcement officers, & other courageous patriots to protect our liberties, freedom, & democracy. May they stay resolute & strong despite corrupt political headwinds they face," Brennan tweeted.

As in previous times of National peril, we rely on our military, diplomats, intelligence officials, law enforcement officers, & other courageous patriots to protect our liberties, freedom, & democracy. May they stay resolute & strong despite corrupt political headwinds they face. https://t.co/QcrbJ0XtQ6 — John O. Brennan (@JohnBrennan) October 29, 2019

Brennan's tweet came in response to a tweet from the president earlier in the day in which Trump questioned whether Lt. Col. Alexander Vindman had been "on the same call that I was," and described Vindman as a "Never Trumper," referring to Republicans who vowed to oppose his election in 2016.

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"Supposedly, according to the Corrupt Media, the Ukraine call 'concerned' today’s Never Trumper witness. Was he on the same call that I was? Can’t be possible! Please ask him to read the Transcript of the call. Witch Hunt!" Trump tweeted.

The tweet was Trump's latest attack on House Democrats' impeachment inquiry, which Trump and his GOP allies have characterized as a partisan attempt to overturn the 2016 election. Democrats have argued that the probe is focused on Trump's attempts to persuade Ukraine's president to investigate former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE, currently a front-runner in the Democratic 2020 primary, and allegations that Trump attempted to withhold military aid over the issue.

Lawmakers are set to vote later this week on the impeachment inquiry for the first time, which Democrats hope will strengthen the House's claims that the investigation is a legitimate probe into the president's alleged wrongdoing.