Brent Scowcroft, the national security advisor to President George H.W. Bush and one of the leading figures of the Republican national security establishment, endorsed Hillary Clinton for president Wednesday.The endorsement is the most high-profile indication yet of the depth of discomfort that Republican foreign policy figures feel about Donald Trump as their party's nominee.The Clinton campaign sees doubts about Trump's ability to handle foreign policy as a major plus for their side. Endorsements by well-known Republican foreign policy figures could add to her advantage on that front."The presidency requires the judgment and knowledge to make tough calls under pressure,"&nbsp;Scowcroft said in a statement in which he did not mention Trump's name. Clinton "has the wisdom and experience to lead our country at this critical time."Scowcroft, 91, served in senior positions under Presidents&nbsp;Ford and&nbsp;Reagan, in addition to his post in the first President&nbsp;Bush's White House. He served as an informal advisor to President George W. Bush, although his public warnings against going to war in Iraq distanced him from the administration.Trump has alienated many Republican foreign policy experts with his criticism of NATO, his seeming embrace of Russian president Vladimir Putin, his trade protectionism and his casual statements about nuclear weapons.&nbsp;The Clinton campaign has largely kept those GOP figures at arm's length so far. Any overt connection with them could worsen tensions with Sen. Bernie Sanders and his supporters. Scowcroft, however, is somewhat less of a lightning rod than others because of his skepticism about Iraq.