US House Speaker Paul Ryan holds a news conference following a private meeting with Republican US presidential candidate Donald Trump on Capitol Hill in Washington, the United States, May 12, 2016. [Photo/Xinhua]

The highest-ranking GOP officeholder's reservations about the party's standard-bearer sent shock waves through the party, and underscored the uphill battle Trump was now facing to repair the fractured GOP in the wake of a chaotic primary season.

Four previous GOP presidential nominees, namely George H.W. Bush, George W. Bush, John McCain and Mitt Romney had already announced that they wouldn't attend the GOP National Convention in July when Trump would be formally nominated.

Trump first drew widespread criticism last June when he said in his presidential announcement speech that Mexico was sending "rapists" and drug dealers to the United States. Since that, he had repeatedly vowed, if elected president, to deport about 11 million undocumented immigrants in the country.

In another outburst of emotional remarks, Trump called for a "total and complete" ban on Muslims entering the United States in the wake of the Paris terrorist attacks in November 2015.