The nation's largest manufacturing trade group slammed the growing influence of proxy advisers on shareholder votes and urged the Securities and Exchange Commission to step up oversight of them, according to a letter reviewed by the Washington Examiner.

The pressure from the National Association of Manufacturers — which counts ExxonMobil, Boeing, Caterpillar, and Microsoft as members — comes as advisory firms Glass Lewis and Institutional Shareholder Services face increasing scrutiny in Washington, D.C.

Lawmakers like Rep. Sean Duffy, R-Wis., are pushing legislation that would force the companies, which suggest how shareholders should vote on matters from executive compensation to board membership and environmental policy, to register with the SEC and disclose any potential conflicts of interest. Critics are also raising questions over Glass Lewis and ISS' role in advancing more politically-motivated proposals and whether they give companies sufficient time to address perceived errors in their recommendations.

The firms have both pushed back strongly against the criticism. ISS on Tuesday blasted a critical report by the nonprofit American Council for Capital Formation as misleading and said it was compiled by "a corporate lobbying group" that has backed the Main Street Investors Coalition, an organization that has openly opposed the advisory firms.

In advance of the SEC's roundtable discussion on proxy advisers on Nov. 15, the manufacturers' group said the agency should "consider a more direct approach" to oversight to address, among other things, the lack of transparency around the process for developing recommendations.

"The current situation is far from a neutral, fact-based process, and the recommendations produced are often problematic in a variety of ways," the group wrote. The manufacturers highlighted the growing use of the shareholder proposal process by activists to push more politically-motivated measures on issues such as firearm sales and diversity.

It's inappropriate to "for activists to abuse the proxy ballot to push goals that are better addressed by Congress or other policymaking institutions," the group wrote, especially when those goals would further a political agenda.

Critics have also questioned the influence on shareholder votes by large pension funds like the California State Teachers' Retirement System, or CalSTRS, which oversees $229 billion in assets. CalSTRS publicly discloses corporate policies it supports, including the appointment of independent directors, rather than company executives, as board chairs and environmentally-sustainable operations.