In an interview focused largely on foreign policy, Trump said defeating Islamic State is a higher priority than persuading Syrian President Bashar al-Assad to step down, playing down a long-held goal of US policy.
Trump questioned how Clinton would negotiate with Russian President Vladimir Putin after demonizing him, and blamed President Barack Obama for a downturn in US relations with the Philippines under its new president, Rodrigo Duterte.
In addition, Trump bemoaned a lack of Republican unity behind his candidacy and said he would easily win the election if the party leaders would support him.
“If we had party unity, we couldn’t lose this election to Hillary Clinton,” he said.
On Syria's civil war, Trump said Clinton could drag the United States into a world war with a more aggressive posture toward resolving the conflict.
Clinton has called for the establishment of a no-fly zone and “safe zones” on the ground to protect non-combatants. Some analysts fear that protecting those zones could bring the United States into direct conflict with Russian warplanes.
"What we should do is focus on ISIS. We should not be focusing on Syria," said Trump as he dined on fried eggs and sausage links at his Trump National Doral golf resort. "You’re going to end up in World War Three over Syria if we listen to Hillary Clinton," he said.
"You’re not fighting Syria any more, you’re fighting Syria, Russia and Iran, all right? Russia is a nuclear country, but a country where the nukes work as opposed to other countries that talk," he said.
Trump said getting Assad to leave power was less important than defeating Islamic State.
"Assad is secondary, to me, to ISIS," he said.
Obama focused 'on his golf game'
On Russia, Trump again knocked Clinton's handling of US-Russian relations while secretary of state and said her harsh criticism of Putin raised questions about "how she is going to go back and negotiate with this man who she has made to be so evil," if she wins the presidency.
On the deterioration of ties with the Philippines, Trump aimed his criticism at Obama, saying the president "wants to focus on his golf game" rather than engage with world leaders.
Since assuming office, Duterte has expressed open hostility towards the United States, rejecting criticism of his violent anti-drug clampdown, using an expletive to describe Obama and telling the United States not to treat his country "like a dog with a leash."
The Obama administration has expressed optimism that the two countries can remain firm allies.
Trump said Duterte's latest comments showed "a lack of respect for our country."
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The interview comes two weeks before the Nov. 8 election, with Trump trailing badly in the polls. He repeated his assertion that the "media is rigging the polls" and said his supporters were upset with the leadership of the Republican Party.
Trump, who has had lukewarm support from House of Representatives Speaker Paul Ryan, the top US elected Republican, was asked whether he thought Ryan should step down, as some of his supporters would like.
"I don't want to make that statement. But I can only say this: If the leaders of this party were pushing for a win, we could not lose the presidential election," Trump said.
He said the base of the Republican Party is on his side, pointing to the large numbers of people who attend his rallies.
"I think those people are very, very disappointed that the leaders aren’t helping us win," he said.
He said if he wins he would not consider putting Democrats in his cabinet but would work with them on legislation.
Trump said during his first 100 days in office he would work to secure the US border with Mexico and start rebuilding the US military and improving care for veterans. He would also get legislation through the US Congress imposing penalties on companies that have left the United States and export their products back to the country.
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