Republican senators are increasingly distancing themselves from the White House amid growing concerns Donald Trump could be facing a "bloodbath" defeat.
Joe Biden's lead over Mr Trump has grown to 9.6 per cent in an average of recent polls, just short of the 10 per cent figure sometimes used to define a "landslide".
Some Republican strategists and donors have begun suggesting a shift in resources to protect vulnerable Senate seats, arguing that keeping the Senate is the best way to put a check on a potential Biden presidency.
Ted Cruz, the Texas senator who was runner-up to Mr Trump in the 2016 Republican presidential primary, said he was "worried" and the election was "highly volatile".
If Americans felt "optimistic" about the pandemic and economy on November 3, Mr Trump could still win by a "big margin", he said.
But he added: “I also think, if on Election Day people are angry and they’ve given up hope and they’re depressed, which is what [the Democrat leadership] want them to be, I think it could be a terrible election.
"I think we could lose the White House and both houses of Congress, that it could be a bloodbath of Watergate proportions."
The Senate is held by Republicans with a 53-47 majority. Of the 100 seats, 35 are being voted on this election, and 23 of those are held by Republicans.
Seats that had been regarded as safely Republican, including in Alaska, Iowa, North Carolina, Kansas and Montana, are now in Democrat sights. Lindsey Graham, a close presidential ally, is now in a tied race in South Carolina.
As they returned to their states to campaign some Republican senators, although not Mr Graham, have been notably cool in their comments about the president.
Mitch McConnell, the Republican leader in the Senate, was starkly critical after attendees at a White House event were diagnosed with coronavirus.
Speaking at a re-election event in his state of Kentucky, Mr McConnell said: "I actually haven't been to the White House since August the 6th because my impression was their approach to how to handle this was different than mine, and what I insisted that we do in the Senate, which is to wear a mask and practice social distancing."
Mr McConnell, 78, who survived polio as a child, sent the Senate into recess after two Republican senators who were at the White House event on September 26, contracted the virus.
The remarks from Mr McConnell were taken by some as a signal other Republican senators were at liberty to criticise the president over the pandemic.
Dr Anthony Fauci, America's top infectious diseases expert, said the Rose Garden ceremony, to nominate judge Amy Coney Barrett as Mr Trump's pick for the Supreme Court, did constitute a "super-spreader event".
He said: "We had a super-spreader event in the White House, and it was in a situation where people were crowded together, were not wearing masks. So the data speak for themselves. I was not surprised to see a super-spreader event given the circumstances."
Tom Tillis, a Republican senator fighting for re-election in North Carolina, who attended the ceremony and later tested positive, told local media in his state he made a mistake taking his mask off.
Mr Tillis said he still thought the president would win but "the best check on a Biden presidency is for Republicans to have a majority in the Senate."
John Cornyn, a veteran Republican senator from Texas, said Mr Trump “let his guard down" on the pandemic. He told the Houston Chronicle: "I think he got out over his skis and, frankly, I think it’s a lesson to all of us that we need to exercise self-discipline."
In Arizona, Martha McSally, a Republican losing the race for John McCain's former senate seat, repeatedly refused to answer if she was "proud" to support the president.
Dan Eberhart, a major Republican donor, said the "Senate map is looking exceedingly grim," while a Republican strategist said Mr Trump was proving an "anchor" in some of the key Senate races, rather than the "buoy" he was in 2016.
Other Republicans, including senator Susan Collins, in a key re-election battle in Maine, criticised the president for not yet agreeing a new pre-election economic stimulus plan with Democrats in Congress.
It came as the president said he was now "medication free" following his treatment, and was no longer taking the steroid dexamethasone.
He was last night due to hold his first in-person event since his diagnosis, speaking from the White House balcony to an audience on the lawn, and will hold a rally in Florida on Monday.
Meanwhile, Chris Christie, the former New Jersey governor, said he had been released from hospital after a week. Mr Christie, who attended the Rose Garden event, was treated for coronavirus.