Biden assured Zelensky during a visit to Washington that strong US support for his war to repel Russian invaders would be maintained despite opposition from some Republican lawmakers.

Some of Trump's Republican allies have called for a defunding of the FBI to protest its investigation into and prosecution of more than 1,140 Trump supporters who stormed the US Capitol to overturn his election defeat.

The House and the Democratic-led Senate have less than two weeks to avoid a shutdown by passing either short-term or long-term funding measures.

The inquiry is centred around whether Biden benefited from Hunter Biden's business dealings.

National opinion polls have long shown overwhelming support among Republicans for another Trump term.

Retiring Gen David Berger formally relinquished command as the Marine Corps' commandant, creating the first of several possible vacancies of Senate confirmed leaders on the US military's Joint Chiefs of Staff.

The debate will be the first chance for voters to see the Republican presidential candidates square off against each other.

They accuse Biden of failing to control immigration.

The number of Republicans who believe the former president is being unfairly targeted vastly exceeds the 30-35% of Trump supporters who are estimated by political analysts to make up his core base.

Biden and McCarthy emerged from a meeting on the debt ceiling talking about the need to find bipartisan compromise, even as they cling to policies that expose the divides between the two parties.

Alarm bells are meanwhile ringing over the possibility of a first-ever US default, with uncertainty over the actual date the government would stop being able to pay its bills.

Another run would test whether voters are ready to give Biden, already the oldest American president ever, another four years in office.