WASHINGTON – A new law will let companies contribute billions of dollars less to their workers’ pension funds, raising concerns about weakening the plans that millions of Americans count on for retirement.

But with many companies already freezing or getting rid of pension plans, many critics are reluctant to force the issue.

Some expect the changes, passed by Congress last month and signed Friday by President Barack Obama, to have little impact on the nation’s $1.9 trillion in estimated pension fund assets. And it is more important, they suggest, to avoid giving employers a new reason to limit or jettison remaining pension benefits by forcing them to contribute more than they say they can manage.