We now know for certain that Russia invaded our democracy. They didn’t use bombs, jets or tanks. Instead, they planned a mission to undermine the foundation of our electoral system. This mission, according to special counsel Robert S. Mueller III, was “sweeping and systematic,” yet the U.S. remains vulnerable to many of the same tactics utilized against us in 2016 and 2018.

This isn’t a partisan issue. In the last presidential election, Russia aimed its attack at the Democratic candidate; next time it could be the Republican. But there are steps we can take to help protect the electoral process.

Russia’s 2016 assault was carefully planned. In 2014, Russian agents landed on U.S. soil in order to gather information — to learn how to mimic us — so that their social media posts would be more believable. They developed a sophisticated network of online personas backed by bots designed to make hateful and divisive posts go viral.

Political ads sold on TV and radio are required to disclose the organizations that paid for them. But right now, the same rules don’t apply to ads sold online.

According to disclosures made to Congress, 126 million Facebook users saw posts linked to Russia. That’s more than a third of the U.S. population. There were also thousands of Russian-sponsored YouTube videos and tens of thousands of tweets aimed at swaying the election. But all of this wasn’t fully understood until it was too late.

And the threat isn’t over. Last month, FBI Director Christopher Wray described the 2018 efforts by Russia to interfere in American elections as “a dress rehearsal for the big show in 2020.” We need to take action now to prevent a recurrence of 2016 — or something even worse.

Currently, political ads sold on TV and radio are required to disclose the organizations that paid for them. This is a simple requirement that the Supreme Court — including the late Justice Antonin Scalia — upheld. But right now, the same rules don’t apply to ads sold online. This leaves a huge loophole in the law — especially because online ads have become more popular than ever. In 2018, an estimated $2.3 billion was spent on online ads, compared with $1.4 billion in 2016, and just $71 million in the 2014 election cycle. For the 2020 election cycle, online ad spending is projected to reach nearly $3 billion.

While some social media companies have taken steps to implement new transparency rules, we need more than a patchwork of company-generated solutions to ensure that political ads purchased by our adversaries are exposed. We need rules of the road that apply to all social media companies.

The bipartisan Honest Ads Act, which I introduced this week with Sens. Lindsey Graham (R-S.C.) and Mark Warner (D-Va.), would shine a light on the dark money being used to buy online political ads.

The goal is simple: to bring our laws into the 21st century to ensure voters know who is paying to influence our political system.

The legislation would achieve this by amending existing laws that now apply to political ads sold in print and on TV and radio, and extend their reach to online political advertising.

The Honest Ads Act would require digital platforms with at least 50 million monthly viewers — which includes major tech companies like Facebook, Google, and Twitter — to maintain a public file of political ads sold on their platform by a person or group who spends more than $500 on political ads in a year. The file would contain a digital copy of the advertisement and key information about who paid for the ad and who the ad was designed to target.

The bill would also require online platforms to do a better job when it comes to making sure that foreign individuals and entities are not purchasing political advertisements in order to influence the American electorate.

A lasting legacy of the late Sen. John McCain is the bipartisan Campaign Finance Reform Act of 2002 — also known as McCain-Feingold — which, among other things, required politicians to say, “I approve this message,” and required more transparency for political ads.

The law was intended to ensure that voters know who is paying to influence our political system.

McCain knew that the next frontier of campaign finance reform would be closing the loopholes that are currently being exploited by foreign adversaries to sow division among Americans. That’s why he was the lead Republican sponsor of the Honest Ads Act when we first introduced this legislation in the last Congress. It didn’t become law in that Congress, but we are hopeful it will now.

At the time, McCain said, “I have long fought to increase transparency and end the corrupting influence of special interests in political campaigns, and I am confident this legislation will modernize existing law to safeguard the integrity of our election system.”

This modernization cannot come soon enough.