Donald Trump suffered another blow to his chances of uniting Republicans behind him, after a Congressman and military veteran said he was unable to support the tycoon’s bid for the presidency.

Republican Congressman Adam Kinzinger, who served in Iraq, said he would not support Hillary Clinton or tell voters how they should vote. But he said he could not support a nominee who “throws all these Republican principles on their head”.

“I’m an American before I’m a Republican,” Mr Kinzinger told CNN. “I won’t be silent. He can tweet all he wants. I have to do this for my country and for my party.

Mr Kinzinger, who was elected to Congress in 2010, has openly hesitated to embrace Mr Trump as his party’s standard-bearer. He said he went to the convention in Cleveland hoping to “at least mildly endorse" the Republican front-runner.

But in the aftermath of Mr Trump’s comments about Russian President Vladimir Putin and the parents of slain Muslim American soldier Humayun Khan, the third-term Illinois congressman said that he did not “see how I get there anymore”.

Usborne on Republican split

“I’m a Republican because I believe that Republicanism is the best way to defend the United States of America,” Mr Kinzinger said on Wednesday evening.

The congressman also said the possibility that Mr Trump could be a drag on races further down the ticket was a factor and that he wanted to focus on defending the Republican majority in the House of Representatives.

Trump is proving hard to love even within his own party (Reuters) (REUTERS)

The high-profile defection is the second of its kind against Mr Trump in two days. Richard Hanna, a Republican congressman from New York who is retiring after this term, announced on Tuesday he would vote for Ms Clinton in November.