US presidential nominee Donald Trump has ratcheted up tensions in his Republican Party by denying leading figures support in their re-election bids, while his Democratic rival Hillary Clinton won her first endorsement from a Republican lawmaker.
President Barack Obama blasted Trump as unfit to be president and questioned why any Republican would support the New York businessman seeking his first public office.
"The question I think that they have to ask themselves is, if you are repeatedly having to say in very strong terms that what he has said is unacceptable: Why are you still endorsing him?" Obama said at a White House news conference with Singapore's Prime Minister, Lee Hsien Loong.
Trump has troubled many in the Republican establishment with his off-the-cuff, often insulting style, and controversial policies including a proposed ban on the entry of Muslims to the US and his plan to build a wall along the Mexican border.
The latest exchanges cast a shadow over the show of unity the party sought to project at the Republican National Convention that formally nominated Trump for president in July.
Interviewed by The Washington Post, Trump said he could not endorse House Speaker Paul Ryan nor Senator John McCain, both of whom face Republican challengers in primary votes in their states ahead of the November 8 election.
Both lawmakers had rebuked Trump over his feud with the family of a slain Muslim-American Army Captain Humayun Khan.
Mirroring the language Ryan used about supporting Trump before his eventual endorsement, Trump told the newspaper he was "not quite there yet" in endorsing Ryan, the highest ranking elected Republican, in next Tuesday's Wisconsin primary.
Trump praised Ryan's opponent, Paul Nehlen, for running "a very good campaign". Trump said Ryan had sought his endorsement, but that as of now he is only "giving it very serious consideration".
Although several Republicans in Congress have said they will not support Trump, Representative Richard Hanna of New York was the first to take the extra step and endorse Secretary of State Clinton, the Democratic nominee.
Hanna, who is retiring from the House of Representatives rather than seek re-election, said his decision was prompted by Trump's attacks on the parents of Captain Khan, who was killed in the line of duty in Iraq in 2004.
The Republican lawmaker called Trump "deeply flawed in endless ways", "unrepentant" and "self-involved".
"For me, it is not enough to simply denounce his comments: He is unfit to serve our party and cannot lead this country," Hanna wrote in a letter posted on syracuse.com, the website of the Post-Standard newspaper in New York.
Obama said Trump's attacks on the Khans showed he was "woefully unprepared" to be president.
Republican Chris Christie, a Trump ally once viewed by Trump as a potential running mate, joined the fray on Tuesday, calling criticisms of the Khan family "inappropriate".
Trump has fallen behind Clinton in opinion polls made public since the parties held their nominating conventions last month.
Clinton extended her lead over Trump to eight percentage points, according to a Reuters/Ipsos poll released on Tuesday, from six points on Friday. About 43 percent of likely voters favour Clinton, 35 percent favour Trump, and 9 percent picked "other".