WASHINGTON -- Almost half of the money spent on campaign ads in the 2018 midterm elections are from groups that keep their donors hidden, and there is no way to know whether corporations are making secret contributions to candidates and causes their customers oppose.

That is, unless the companies themselves disclose their donations.

A growing number of publicly traded corporations are doing just that, led by Becton, Dickinson & Co., of Franklin Lakes, one of three companies who received the highest score in the annual Index of Corporate Political Disclosure and Accountability produced by the Center for Political Accountability and the University of Pennsylvania Wharton School's Zicklin Center for Business Ethics Research.

"They recognize that political spending does post serious risks to them," said Bruce Freed, director of the Center for Political Accountability, a Washington-based advocacy group pushing for more corporate disclosure of political spending.

Of the 414 companies in the S&P 500 since 2015, 137 either fully disclose or prohibit donations to trade associations for political use, up from 97 in 2 015. And 119 fully disclose or prohibit contributions to nonprofit groups that can be spent on politics, an increase from 81 three years ago.

"The public, various stakeholders, even company employees look at the company's political spending and react to it if it conflicts with company core values, company company positions, company products," Freed said. "The company takes a big reputational hit and a business hit on that."

Four other New Jersey-based companies, Merck & Co., Celgene Corp., Johnson & Johnson and Prudential Financial, also scored high in the index.

Former Celgene executive Bob Hugin is the Republican nominee against U.S. Sen. Robert Menendez, D-N.J., this fall.

Celgene Executive Vice President Richard Bagger said the rating "reflects both our ongoing commitment to transparency and our corporate values, which emphasize trust in our words and actions. We are proud to have been on the forefront of political contributions reporting and will continue providing this information in twice-annual public reports."

Becton Dickinson did not respond to a request for comment.

Political spending by trade associations and nonprofit groups, which can keep their donors hidden, has grown since the U.S. Supreme Court's five-member conservative majority struck down limits on corporate and union spending in its 2010 Citizens United decision.

Through Sept. 17, 49 percent of the money spent on campaign ads during the midterms, seen as a referendum on President Donald Trump, came from groups that do not disclose their donors, according to the Wesleyan Media Project, which is tracking such spending.

Even some political groups that do disclose the sources of their contributions, such as the Congressional Leadership Fund super political action committee linked to House Speaker Paul Ryan, get millions of dollars from nonprofits whose donors remain hidden.

Hence, the ongoing efforts to get companies to disclose whether they're spending corporate funds on behalf of candidates or causes that may be at odds with their businesses or customers.

Merck and Johnson & Johnson, two of the biggest sellers of contraceptives, and their trade association gave millions of dollars to support Republican governors and legislatures that tried to block access to birth control.

And both Merck and Comcast Corp., who supported Cory Booker's run for the Senate through their PACs, also contributed to trade associations giving to a nonprofit backing his Republican opponent.

The classic example is Target Corp., which faced a boycott by gay-rights groups in 2010 after donating $150,000 to a business group backing a Minnesota gubernatorial candidate who opposed same-sex marriage.

Jonathan D. Salant may be reached at jsalant@njadvancemedia.com. Follow him on Twitter @JDSalant or on Facebook. Find NJ.com Politics on Facebook.