One of the market's most important players is losing its mojo.

Corporate America's share buybacks, which hit a record of more than $1 trillion in 2018 on the back of a tax overhaul, have begun to slow down in recent months, posing a threat to the record-long bull market, according to Jefferies.

"Total S&P 500 buyback amounts seem to have peaked while the share of R&D spending has remained constant since the Great Financial Crisis," Sean Darby, global head of equity strategy at Jefferies, said in a note Wednesday.

In the second quarter, S&P 500 share buybacks totaled $160 million, about 20% less than the first quarter and also year over year, according to Jefferies. The utilities and health-care sectors saw the biggest decline in buyback activities in the second quarter, the bank noted.

Share repurchases have long been a pillar of support for the broad market. By reducing the number of outstanding shares in the market, companies are able to boost stock prices and earnings per share figures. A slowdown in buybacks could be an ominous sign for the market which has already been threatened by an escalating trade war and slowing growth.

The S&P 500 endured a volatile third quarter, returning 1.7% amid tit-for-tat tariffs and impeachment fears. The fourth quarter was off to a tough start with the S&P 500 losing 1.2% on Tuesday, hit by a slump in the U.S. manufacturing sector. Stocks were down big again Wednesday.