By Brendan O'Brien

MILWAUKEE (Reuters) - U.S. House of Representatives Speaker Paul Ryan faces a primary for his congressional seat on Tuesday in a race overshadowed by presidential candidate Donald Trump's brief refusal last week to endorse his fellow Republican.

Ryan, who ran unsuccessfully for vice president in 2012, is predicted to easily beat challenger Paul Nehlen by 66 points in the Republican primary contest ahead of the Nov. 8 general election, a Remington Research Group poll showed last week.

Despite the expected large margin of victory, the race for the 1st Congressional District in southeast Wisconsin became the center of attention last Tuesday when Trump refused to endorse Ryan during an interview with the Washington Post.

In a sign of the tension between the politicians, Trump told the newspaper he was "not quite there yet" - nearly the same phrase Ryan had used about Trump.

On Friday, Trump endorsed Ryan and Senators John McCain of Arizona and Kelly Ayotte of New Hampshire during a campaign stop in Green Bay, a show of support that could be a step to mend his frayed relations with fellow Republicans.

Trump, a former reality TV star, has troubled many in the Republican establishment with his off-the-cuff, often insulting style, and controversial policies, including the proposed ban on Muslims visiting the United States and his plan to build a wall along the Mexican border to keep out illegal immigrants.

Ryan, Ayotte and McCain had criticized Trump's feud with the family of Army Captain Humayun Khan, who died in the line of duty in Iraq in 2004 and was awarded the Bronze Star Medal for bravery after his death. Trump has had a running dispute with Khan's parents since they criticized Trump at last month's Democratic National Convention.

(Reporting by Brendan O'Brien in Milwaukee; Editing by Gareth Jones)