Stock buybacks are back. Companies in the S&P 500 repurchased $234.5 billion in shares during the third quarter of 2021, topping the previous record of $223 billion in the fourth quarter of 2018, according to preliminary data from S&P Dow Jones Indices. The wave of share repurchases has helped propel U.S. stock indexes to dozens of records in 2021.

The S&P 500 is up 25% in 2021, notching 67 record closes.  Why are repurchases believed to have been one of the reasons for the stock market's strong performance in 2021? Why are some politicians critical of corporations' share repurchase activities?

What other factors referenced in the market are considered to have propelled share prices upwards?