U.S. Presidential Candidates Hillary Clinton, left, and Donald Trump
U.S. Presidential Candidates Hillary Clinton, left, and Donald Trump

President Barack Obama said Thursday that his administration was required by law to give national security briefings, including classified information that is not disclosed to the public, to major presidential nominees.

This includes Republican Donald Trump, whom Obama has called unfit to serve as president.

During the past year of political campaigning, Trump has become known as a public speaker who often ignores prepared texts and relies instead on spontaneous remarks that are sometimes seen as unusually frank or critical of other American public figures. That has prompted concern in some quarters about how Trump would treat information gained from national security briefings.

"If they want to be president," Obama, speaking of party nominees, told reporters at the Pentagon, "they've got to start acting like a president, and that means being able to receive these briefings and not spread them around."

The main reason for the nominees' briefings, Obama said, is to ensure that a president-elect, whether a Democrat or a Republican, does not step into the job unprepared.

'Rigged' election

Trump, meanwhile, has told supporters at campaign rallies that he is concerned the U.S. election in November will be "rigged" against him. Obama dismissed that claim Thursday as "ridiculous."

FILE - U.S. military veterans (R-L) Perry O'Brien,
FILE - U.S. military veterans (R-L) Perry O'Brien, Claude Copeland and Julio Torres demonstrate outside Republican presidential candidate Donald Trump's news conference outside Trump Tower in New York, May 31, 2016.

In a separate development, a group of U.S. military veterans delivered petitions to Arizona Senator John McCain and other leading Republicans in Washington on Thursday, urging them to withdraw their public endorsements of Trump's candidacy.

Former Marine Alexander McCoy said he was shocked by Trump's "reckless ignorance about America's responsibilities in the world."

Navy veteran Nate Terani, who is a Muslim, decried what he called the Republican nominee's "hate speech, bigotry and unabashed incitement to violence against minorities," including Muslims.

Trump's seemingly lighthearted acceptance this week of a gift of a Purple Heart medal — awarded to wounded or dead service members — and his public feud with Khizir and Ghazala Khan, the parents of a Muslim American soldier killed in Iraq in 2004, have dismayed many American military veterans, who have complained they felt insulted and dishonored by the candidate's remarks.

Endorsement stands

McCain, a Vietnam War veteran and former prisoner of war, has said Trump's remarks about the Khans do not represent the Republican Party, but he has declined to rescind his endorsement of Trump as the party's chosen candidate for president.

Campaigning Thursday in Portland, Maine, Trump promised to take care of veterans who still endure long waits for government-provided medical care, and to strengthen what he called a "depleted military."

Trump said his main opponent, Democrat Hillary Clinton, could not be trusted with national security matters. He contends she put the country at risk by the way in which she handled classified information in email exchanges when she was secretary of state.

U.S. Democratic presidential nominee Hillary Clint
FILE - U.S. Democratic presidential nominee Hillary Clinton holds up a Donald Trump-brand tie made in China during a stop at a local tie company in Denver, Colorado, Aug. 3, 2016.

Clinton was in Las Vegas, Nevada, on Thursday, visiting an electrical parts plant. The Democratic candidate noted again that virtually all of the many Trump-branded products on sale in the U.S., such as shirts, ties and other garments, are manufactured abroad, not in the United States.

'Look at what he's done'

"Everything he's made, he's made somewhere else. He's not putting Americans to work," Clinton said. She added that Trump hires foreign workers to fill low-paid jobs at his country clubs and has frequently failed to pay contractors who worked on his projects.

"I've met people who were destroyed by Donald Trump," Clinton said, "so take a look at what he's done, not what he says."

On Friday, Trump is due to appear with his vice presidential running mate, Indiana Governor Mike Pence, in Des Moines, Iowa. Clinton is scheduled to campaign in New York and in McLean, Virginia, just outside the U.S. capital.

Recent surveys of American voters indicated Clinton was leading Trump by an average of 6 percentage points nationally. A poll released this week by Fox News estimated the Democratic candidate was ahead of Trump by an even wider margin, 49 percent to 39 percent; others taking part in the survey either were undecided or planned to vote for other candidates.

VOA's Michael Bowman, Ken Bredemeier and Jim Malone contributed to this report.