The Senate voted on Friday to override President Donald Trump's veto on the defense spending bill in a rare New Year's Day session.

The stinging rebuke by members of Trump's own party represents the first time in his term that a veto has been overturned.

The vote was 81-13. A supermajority is needed to override a presidential veto.

Sen. Mitt Romney, R-Utah, confirmed today that Mitch McConnell, the Senate majority leader, called the upcoming joint session of Congress on Jan. 6, which will affirm the results of the presidential election, "the most consequential vote" of his lengthy tenure.

"I see that as a statement that he believes it's a -- it's a referendum on our democracy," Romney told reporters.

Sources said that McConnell asked Sen. Josh Hawley, R-Mo., on a New Year's Eve call with Republicans to explain to his colleagues why he is planning to object to the certification of Biden’s electoral win during the joint session. McConnell had privately warned his colleagues weeks ago against doing this as it would put his conference in the position of having to oppose Trump (and thereby his base) publicly.

Hawley, in joining the last-ditch bid by Trump's House allies to overturn the election results, said he objected to states not following their election laws.

"At the very least, Congress should investigate allegations of voter fraud and adopt measures to secure the integrity of our elections," Hawley said in a statement. There has been no evidence of widespread election fraud.

-ABC News' Trish Turner

In a very rare New Year’s Day session, Senate GOP leadership rejected two attempts to debate and vote on the House-passed CASH Act, which would give most Americans $2,000 in direct COVID relief payments. It was an effort by both Sens. Chuck Schumer and Bernie Sanders.

Republicans are continuing to argue that the House bill —- which would allow those making up to $350,000 to receive some cash under the act, albeit a smaller amount than those making less -- amounts to "socialism for the rich."

The chamber’s socialist, Sen. Sanders -- continued to argue that Republicans were “hypocrites” —- blocking this effort but approving big tax breaks for the rich.

He was joined by conservative Sen. Josh Hawley, who slammed the fight by his own leadership.

“With all due respect, this doesn’t seem to be Republicans against Democrats. This seems to be the Senate against United States of America,” said Hawley.

-ABC News' Trish Turner

The Senate is set to cast a preliminary vote this afternoon on overriding President Trump's veto of the National Defense Authorization Act -- which would be the first of his term.

The House voted overwhelmingly earlier this week to override Trump's veto of the $740 billion legislation, which funds the military and that members from both parties have said is vital to both the troops, who get a raise under the measure, and national security.

Trump has called the bill a "gift" to China and Russia.

In vetoing the measure (which has passed every year for 59 years), Trump demanded that Congress include a repeal of Section 230 of the Communications Decency Act, which gives broad legal immunity to tech companies over contents posted by users. He also wanted lawmakers to nix a provision authored by Sen. Elizabeth Warren that requires DOD to rename military bases and installations that honor Confederate generals.

Congress originally passed the measure with veto-proof majorities, and the House recently voted to override the veto. If the Senate votes to override - as it is expected to -- the bill would become law.

Today's test vote (which should be similar - if not identical - to the final vote) is expected to show more than the two-thirds majority required to override Trump's veto.

-ABC News' Trish Turner

Following a briefing with his COVID-19 advisory team on Tuesday, Biden is expected to continue his criticism of the Trump administration in afternoon remarks on the coronavirus pandemic from Wilmington, Delaware, and home in on what his team has characterized as the lagging pace of vaccine distribution.

According to a transition official, Biden will provide an update on the current state of the pandemic and give a dire prediction that, tragically, infection and death rates will rise in the coming weeks. Biden is also expected to address the current administration falling short on its pace for vaccinations, the official said, as well as his plan to get 100 million vaccinations in 100 days.

Biden on Monday slammed the Trump administration for, he said, leaving security agencies "hollowed out" and for creating "roadblocks" for his transition team seeking access to key national security information -- a comment that acting Defense Secretary Christopher Miller has pushed back on, saying the Biden team has already been given more interviews and documents than they initially requested.

Vice President-elect Kamala Harris and her husband, Doug Emhoff, on Tuesday morning are slated to receive their first doses of the COVID-19 vaccine in Washington, D.C., where Harris may also comment on the incoming administration's vaccine distribution plans.

Trump, meanwhile, continued to contest the election on Twitter Tuesday morning -- with three weeks until Biden's inauguration -- criticizing Republican leadership as "pathetic," saying they "only want the path of least resistance" and "only know how to lose." He also attacked Republican lawmakers over their handling of the defensing funding bill as the Senate is expected to override Trump's presidential veto for the first time in his presidency by the end of the week.

After Trump changed course on Sunday and signed the coronavirus relief and government funding bill to avert a shutdown -- though not in time to prevent unemployment benefits to millions of Americans from lapsing -- the House of Representatives on Monday approved Trump's call to increase the coronavirus relief payments from $600 to $2,000.

But its fate in the Senate is uncertain. Senate Majority Leader McConnell has declined to reveal how he will proceed or even whether he’ll bring the proposal up for a vote. Democrats are seizing the opportunity to force Republicans into a difficult position -- either backing or defying Trump.

-ABC News' Molly Nagle and Ben Gittleson