Democrats repeated it during GWB's term as well in 2004, 2006, 2008, after slow rolling all of his nomination for 8 years and inventing new ways to delay the process and not do their job. Democrats were applying this new "rule" to the end of ANY term (including mid terms), not lame duck's at the end of their second term.  They promised we would wait for after the election and see how the democracy had played out.
Well, when Antonin Scalia died at the end of Barack Obama's second term, with Obama sitting at historically low popularity, and an opposition party in power in both House and Senate, and the Republicans applied the Biden rule back at the Democrats, with more justification than Biden had when he invented it. The far left media and democrats contested that, "this was unheard of", "How dare they... just because the Democrats had promised it, multiple times during the prior three Republican election years", and so on. But the media and the Administration didn't fight too hard, because they were assured that Hillary's election was "in the bag", and she would put in a more far left appointment -- maybe even Barack Obama. (It was rumored that was the back-room trade for him supporting her).
Despite the Democrats protests, this is not really as unusual as it sounds, in fact it is kind of the norm.
Compared to 113 successful appointments to the Supreme Court, there were 30 that were never confirmed. So it's 25-30% of all appointments, and virtually all late term appointments (by the opposition) was common long before Biden announced to everyone that we were still doing it.
To understand how partisan this was, of the 7 Democratic Presidents elected after 1900, not one had a nominee rejected by Republicans in a Senate vote. Conversely, of the 12 Republican Presidents during the same time, they had 10 of their nominations successfully blocked by Democrats, and a few others brutally maligned in failed attempts to block them.
This list includes:
- Pierce Butler was not allowed out of committee during an election year, but was later confirmed the following year when Harding won.
- John Parker was blocked during an election year.
- John Harlan was blocked in 1954, but was confirmed after the election in 1955.
- Clement Haynsworth, Harold Carswell, Herschel Friday, Mildred Lillie were all blocked by Democrats because they didn't like Nixon.
- Robert Bork, Douglas Ginsburg, were attacked and blocked under Reagan -- in fact, Kennedy (who Kavanaugh just replaced) was Reagan's 3rd choice after Dems blocked the first two. And Clarence Thomas was attacked, but succeeded in spite of the hate campaign against him.
- Most recently Harriet Miers was blocked under George W Bush, and that wasn't even an election year.
Heck the Democrats in the Senate even blocked Lyndon Johnson's pick of Abe Fortas, during an election year, and he never got a vote and later withdrew. and that's just Democrats in the 20th century. Examples of earlier cases where the Senate blocked appointments in an election year (by unpopular President, when the opposition party held the Senate) include:
- John C. Spencer, Reuben Walworth (3 times), and Edward King (twice) appointments by John Tyler. But despite two of them being nominated in January of 1844, the Senate just waited out the clock and refuse to confirm.
- There was also Edward A. Bradford by unpopular President Millard Fillmore which was pretty much the same thing.
Mahlon Pitney was the last Supreme Court Justice appointed in an election year... but that was in 1912, and it was the end of Taft's first term in 1912, Taft wasn't unpopular and went on to win re-election, and the appointment was still opposed by the progressives, they just didn't have enough votes to block him.
The first and only justice to be confirmed late in the second term (while the opposition party controlled the Senate) was Lucius Lamar II in 1888. Grover Cleveland (D) was a popular enough President who went on to be re-elected. While Republicans held the Senate, and they appointed Lucius anyways. Of course it was January (so the election wasn't really going yet), it wasn't at the end of Grover's second term, Lucius had been in congress for many decades so the people appointing him already knew him personally and professionally. So it's not really the same context. And it certainly hadn't happened since Ted Kennedy and the Democrats had been politicizing the court in the 1980's.
In the end, you can only find one example of it happening, and you have to go back 128 years to find it, and even then the conditions weren't the same (popular, 1st term, earlier in the election cycle, worked in Congress) -- then it's not exactly a common practice. It is more accurate to say that never in our history has an unpopular opposition candidate appointed a Supreme Court Justice in their final year. Ever.
The big lie - Of course the far left media outlets like Politifact, Vox, WaPo and the Democrats all lied about it (spun it). They pretended it was fairly common to put justices in, in this situation. They lied by twisting words or skipping context. They pointed out Eisenhower did a recess appointment (not the same thing), they use "election year" appointments which includes mid-term or at the end of their first term, not second term appointments. They also use popular Presidents. Most of all, they omit that the only time it really happened was when the Presidents party also controlled the senate and just overrode the opposition to the appointment. So it's not the same thing at all.  Thus is how the leftist Press/Media makes their readers/viewer dumber and more angry, by feeding them skewed histories.
One of my inconsolable, irrational, #NeverTrump, #Resist, Democrat friends (I know, that's redundant), was screaming that this was unfair -- and Merrick Garland should have been appointed according to the rules. I said, "Sure, maybe he should have". Of course since he's Democrat, he didn't listen and continued his caterwauling, and I repeated myself a couple more times until he started to listen. Then:
- I pointed out that even if what the Republicans did wasn't right -- it was because of the Democrats that it happened. If you threaten and point a gun and threaten to shoot the other side often enough, they might take you seriously and shoot you back/first -- that's just life.
- I'll leave it to the courts or public opinion to decide if that was right or not, but the left denying they had any responsibility in what happened, is just silly. They started it, provoked it, and are not just mad that they lost. It may not be right, but there was a lesson to be learned, and it's don't threaten or waive guns around.
- that's before we get into the fact that they only reason Republicans were able to do that, was because Democrats went along with it, because they thought they'd get to parlay that into an even MORE extreme appointment (like Obama), and they only really pushed back after it was too late, and they'd lost. That's more like crying that they lost an illegal bet they placed. Sure, they might not have been completely legal to make that bet, but we all know they would have been OK with it, if they had won. So their crying is just hypocrisy and whining of losers.
- This might not be what the founders intended, but it has been going on for 200+ years. And there has NEVER been a Supreme Court appointment in these circumstances (despite the media/left's deceit). Never has an unpopular opposition parties Supreme Court nomination been confirmed in a lame-duck election year.
- Lastly, the Democrats have behaved far, far worse when it comes to politicizing the Supreme Court nomination process. So the gross majority of Democrats whining about this are either liars, fools, or both.
This doesn't make the Republicans saints, but if you're following the politicization of the nomination process, the Democrats getting their comeuppance, once, in Merrick Garland was a long time in coming. And their hypocritical whining about it, just shows they're both ignorant of history, dishonest and poor losers.