More than 300 former national security officials — from the Bush administration to the Obama White House — have urged Congress to hold President Trump accountable for his call with Ukrainian President Volodymyr Zelensky, a conversation in which he pressured the foreign leader to investigate a political opponent. They wrote in a letter published by the group National Security Action on Friday that they consider the president's actions during that call to be a "profound national security concern."

"President Trump appears to have leveraged the authority and resources of the highest office in the land to invite additional foreign interference into our democratic processes. That would constitute an unconscionable abuse of power. It also would represent an effort to subordinate America's national interests -- and those of our closest allies and partners -- to the president's personal political interest," the bipartisan group wrote.

Read the letter here:

The group suggested that any effort to thwart U.S. interests on the global stage based on personal gain could make the country "more vulnerable to threats, and sends a message to leaders around the world that America's foreign policy can be dangerously corrupted by catering to a single individual."

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They added, "If we fail to speak up — and act — now our foreign policy and national security will officially be on offer to those who can most effectively fulfill the President's personal prerogatives."

The group also encouraged Congress' efforts in pursuing an impeachment inquiry against the president in light of the White House's efforts to squash details of the call.

"All of us recognize the imperative of formal impeachment proceedings to ascertain additional facts and weigh the consequences of what we have learned and what may still emerge. We applaud those Members of Congress, including Speaker Pelosi, who have now started us down that necessary path," they wrote.

They warned, " There is no escaping that what we already know is serious enough to merit impeachment proceedings. From there, the facts -- and nothing but the facts -- should dictate how Congress holds the president to account and signals to the world that our foreign policy and national security are not for sale."

The letter comes just one day after Acting Director of National Intelligence Joseph Maguire testified that an intelligence whistleblower "did the right thing" by filing a complaint against the president.

In a detailed, nine-page complaint, the whistleblower said "senior White House officials" worked to "lock down" all records of the presidential phone call this summer, out of fear that "they had witnessed the president abuse his office for personal gain."