Former presidents Barack Obama and George W Bush are to lead mourners at a service for Senator John McCain at Washington’s National Cathedral.
The Vietnam War hero, who became one of America’s most high-profile politicians, died a week ago from brain cancer at the age of 81.
President Donald Trump will not be attending the memorial service.
The two Republicans had major differences, both personal and political.
McCain’s family made clear Mr Trump was not welcome at memorial services in the state of Arizona – which McCain represented as senator – and Washington, or at Sunday’s private burial service in Annapolis, Maryland, at the US Naval Academy.
Former presidents Obama and Bush – each of whom beat McCain in his twin presidential campaigns – will deliver eulogies, and so will former Secretary of State Henry Kissinger.
McCain’s body was taken from his home to a church in Phoenix, Arizona, on Thursday for a commemoration led by former US Vice-President Joe Biden.
It was then taken to Washington to lie in state at the Capitol.
On the way to the cathedral, the cortege stopped at the Vietnam Veterans Memorial, where McCain’s wife Cindy laid a wreath.
Before he went into politics, McCain was a US Navy pilot who was shot down over Hanoi while on a bombing mission in 1967.
He was held as a prisoner until 1973, enduring torture that – along with the injuries he sustained bailing out of his jet – left him with life-long injuries.
While deeply conservative on some issues, the Arizona senator had a maverick streak that endeared him to political friends and opponents.
He championed reform of US immigration, campaign finance and environmental laws, and was outspoken in criticising those who advocated what he considered torture against captured enemies of the US.