https://www.manhattancontrarian.com/blog/2021-1-12-its-time-to-get-to-know-joe-
For the past four years, the Republicans have held voting control of the U.S. Senate, but by very narrow margins. Since 2017, the Republicans have had either 51, 52 or 53 seats, out of 100, at various times. That has meant that on any given vote, a handful of Republican defectors would be sufficient to sink whatever initiative was on the table. And thus a very small group of independent-minded Republican Senators — most prominently, Susan Collins of Maine, Lisa Murkowski of Alaska, Mitt Romney of Utah, and (until his death in 2018) John McCain of Arizona — held the keys to any Republican plans to get legislation enacted or government officers confirmed.
As a prime example, who can forget the Hamlet act of Senator Collins in the run-up to the Kavanaugh confirmation?
Now, the Senate is going to be split right down the middle, 50/50, with Vice President Kamala Harris having the ability to break a tie when everyone shows up and the vote goes along party lines. Suddenly, the question of whether there might be a couple of potential defectors on the Democratic side — or maybe even just one — becomes significant.
The situation of an even split in the Senate, with the VP breaking the tie, has occurred a few times in U.S. history. For example, one such time was during the presidency of Dwight Eisenhower in the 1950s, when the tie-breaking VP was Richard Nixon. Most recently there was a 50/50 Senate split in 2001, after the very close 2000 presidential election. VP Dick Cheney had the swing vote; but that only lasted for a few months. In May 2001 a Republican Senator from Vermont named Jim Jeffords decided to switch parties and become an “independent,” however “caucusing” with the Democrats. That act immediately swung the balance of power in the Senate to the Democrats.