Facebook, Twitter and Google on Tuesday all declined to endorse a bill intended to bring more transparency to online political ads on their platforms.

"My first question is simply will you support our bill?" Klobuchar asked.

ADVERTISEMENT The companies all responded by saying that they were willing to work with lawmakers on legislation that brings more transparency to the ad process, but none of the representatives were willing to endorse the current bill.

“We certainly support the goals of the legislation and would like to work through the nuances to make it work for all of us,” said Richard Salgado, Google's director of law enforcement and information security.

Facebook General Counsel Colin Stretch said that the company has "drawn on much of what’s in the bill" in crafting its own reforms to its disclosure rules.

"We’re not waiting for legislation," he said.

Klobuchar noted that the companies had changed their policies in recent weeks, but pointed out that they don't have to answer to any independent monitors to ensure compliance.