Given its rightward bent, the chamber has always been more closely aligned with Republicans, though it typically tried to identify some Democrats it could get behind to demonstrate it was receptive to both parties.

But in recent years, the chamber has come to support Republicans almost exclusively. The watchdog group Public Citizen found that in 2016, the chamber devoted all of its campaign money — nearly $30 million — to Republican candidates in primary and general elections, making it one of the largest outside players in congressional races.

The numbers of more moderate Democrats have been decreasing as the party moved left. But even centrist Democrats who consider themselves business friendly found it increasingly hard to meet the standard for an endorsement from the chamber, which can translate into dollars, campaign support and trickle-down backing from businesses back home. The unlikelihood of winning the group’s support discouraged some Democrats who might have welcomed it from even trying to do so.

The chamber endorsed 38 Democrats in 2008, but that number steadily declined to single digits until the group backed just five in 2016. The number ticked up to seven last year as the chamber stepped up its efforts to find Democrats it could back. In contrast, the chamber endorsed 191 Republicans in 2018, a not atypical number.

The sharp imbalance prompted a rethinking at the chamber, where officials attribute part of the problem to the shifting dynamics of Congress. Compared to past years, there are fewer votes in the House and Senate, and the ones that occur are often tests of party loyalty, providing few opportunities for lawmakers to demonstrate independence.

“It used to be a little easier to get votes on policies,” said Neil Bradley, an executive vice president at the chamber and a former top Republican leadership adviser on Capitol Hill. “We had a lot more open amendment process. There were opportunities for folks to demonstrate where they supported the business community. We are down in both bodies to binary choices that largely fall along partisan lines.”

As a result, he and Mr. Reed decided the old system of basing endorsements solely on a willingness to vote the chamber position at least 70 percent of the time was not giving lawmakers credit for other bipartisan activity.