Unbound delegates in the party are free to support the candidate of their choice.
共和黨內無派別代表可以自己自由選擇、支持候選人。
If his nomination is confirmed, Mr Trump will face former US Secretary of State Hillary Clinton or Vermont Senator Bernie Sanders, who are vying for the Democrat nomination.
On Wednesday, the New York billionaire suggested going against Mr Sanders in a TV debate in California before the state’s primary on 7 June.
本周三,川普建議在6月7日舉行加利福利亞州初選前,和桑德斯在加州舉行一場電視辯論。
Mr Sanders agreed to the debate in a tweet, saying "Game on".
桑德斯在推特上留言表示接受這一辯論,并說“游戲開始了”。
On Thursday, Mr Trump said: "The problem with debating Bernie? He’s going to lose."
本周四,川普說道:“和伯尼(桑德斯)辯論有什么問題?他肯定會輸的。”
He also threw a barb in Mrs Clinton’s direction, saying: "Here I am watching Hillary fight and she can’t close the deal. That should be such an easy deal to close."
Earlier, the current US president Barack Obama said that world leaders "had good reason to be rattled" by Mr Trump, whose proposals he said were "either ignorance of world affairs or a cavalier attitude".