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

Just 16 days before he's set to leave office, President Donald Trump will headline a rally for Sens. Kelly Loeffler and David Perdue in Dalton, Georgia.

The rally comes on Jan. 4, the night before the two sitting senators face separate Democratic opponents -- Rev. Raphael Warnock and Jon Ossoff, respectively -- in two runoffs that could determine control of the Senate. The makeup of the Senate is currently projected to be 50 Republicans and 48 Democrats and left-leaning independents. If Warnock and Ossoff win, pushing the balance to 50-50, the Democrats would control both the House and Senate, due to Kamala Harris casting the deciding vote in the upper chamber.

Dalton is in Georgia's 14th Congressional District, which makes up the northwest corner of the state and is perhaps the state's most conservative district. Its congresswoman-elect, Marjorie Taylor Greene, has supported QAnon conspiracies in the past and has fully embraced, defended and perpetuated Trump's false narrative about the election.

The 14th Congressional District is also doing the worst of the 14 districts in terms of turnout for the runoff, according to Georgia Votes, which is analyzing the secretary of state's data.

Trump made his one and only appearance in Georgia during the runoff campaign on Dec. 5, when he spent nearly two hours ticking off baseless conspiracies about the presidential election and went on a tangent about how he didn't want to come to appear at the rally.

-ABC News' Quinn Scanlan