A veteran Republican strategist now helping lead a “super PAC” in support of Donald J. Trump argued in a confidential memo last year that it would be dangerous to give Mr. Trump the power to launch a global war.

The strategist, Alex Castellanos, has acknowledged that he had been publicly critical of Mr. Trump, but said on Sunday that he now considered Mr. Trump a superior choice to Hillary Clinton.

In a private document presented to top Republican donors last fall, however, Mr. Castellanos suggested that Mr. Trump could be a threat to American security as president. He and another strategist, Gail Gitcho, drew up plans for a group named ProtectUS, and proposed to spend heavily against Mr. Trump in the Republican primaries.

In the memo, Mr. Castellanos proposed to urge voters not to put Mr. Trump “in the Big Chair in the Oval Office, with responsibilities for worldwide confrontation at his fingertips.” He also intended to highlight Mr. Trump’s past bankruptcies and predicted that his nomination would be ruinous for Republicans.