Donald Trump has called for "extreme vetting" of immigrants seeking admission to the United States, vowing to block those who sympathise with extremist groups or don't embrace American values.
He said on Monday that the policy would first require a temporary halt in immigration from dangerous regions of the world.
Speaking in swing state Ohio, Trump also said his Democratic rival Hillary Clinton lacks the "mental and physical stamina" to take on the Islamic State.
He said destroying the terror group would be the centrepiece of his foreign policy and he would partner with any countries that share that goal - specifically singling out Russia as a nation the US could have a better relationship with.
"Any country that shares this goal will be our allies," Trump said. "We can never choose our friends, but we can never fail to recognise our enemies."
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Ahead of Trump's address, Clinton and Vice President Joe Biden vigorously challenged the Republican nominee's preparedness to be commander in chief.
Biden called Trump's views "dangerous" and "un-American" and warned that Trump's false assertions last week about President Obama founding the Islamic State could be used by extremists to target American service members in Iraq.
"The threat to their life has gone up a couple clicks," he said.
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While Trump has been harshly critical of Obama's handling of the threat posed by the Islamic State, his own policies for defeating the group remain vague. His most specific prescriptions centred on changing US immigration policy to keep potential attackers from entering the country.
Trump's campaign aides said the new ideological test for admission to the US would vet applicants for their stance on issues like religious freedom, gender equality and gay rights.
The government would use questionnaires, social media, interviews with friends and family or other means to determine if applicants support American values like tolerance and pluralism.
The US would stop issuing visas in any case where it cannot perform adequate screenings.
Trump did not clarify how US officials would assess the veracity of responses to the questionnaires or how much manpower it would require to complete such arduous vetting.
Nor did the campaign say whether additional screenings would apply to the millions of tourists who spend billions of dollars visiting the United States each year.
The Republican nominee's foreign policy address comes during a rocky stretch for his campaign.
He's struggled to stay on message and has consistently overshadowed his policy rollouts, including an economic speech last week, with provocative statements, including his comments falsely declaring that Obama was the "founder" of the Islamic State.
