Democratic presidential candidate Joe Biden said Monday that he is putting Russia and other foreign governments “on notice” that he would act aggressively as president to counter any interference in U.S. elections. The statement came hours after Democratic leaders issued a new warning that Congress appears to be the target of a foreign interference campaign.

Biden said in a statement that he would treat foreign interference as an “an adversarial act that significantly affects the relationship between the United States and the interfering nation’s government.” He criticized President Donald Trump for not doing enough in response to U.S. intelligence agencies’ assessment that Russia interfered in the 2016 presidential election.

“If any foreign power recklessly chooses to interfere in our democracy, I will not hesitate to respond as president to impose substantial and lasting costs,” Biden said.

The new alarms give a renewed urgency to concerns that foreign actors could be trying to influence the vote or sow disinformation. Biden said last week that he had begun receiving intelligence briefings and warned that Russia, China and other adversaries were attempting to undermine the presidential election. Biden gave no evidence, but he said that Russia was “still engaged” after 2016 and that China was also involved in efforts to sow doubts in the American electoral process.

During an online fundraiser Monday night, Biden added: “It’s going to be tough, there’s not much I can do about it now except talk about it, and expose it, but it is a serious concern. It is truly a violation of our sovereignty.”

And in an interview on MSNBC, Biden said Russian President Vladimir Putin “knows I mean what I say.”

The Democratic leaders said in a letter to FBI Director Christopher Wray on Monday that they are concerned that Congress appears to be the target of a “concerted foreign interference campaign” to influence the 2020 presidential election. They asked Wray for an all-members, classified briefing on the matter before the August recess.

The letter from House Speaker Nancy Pelosi, Senate Democratic Leader Chuck Schumer and the top Democrats on the House and Senate intelligence committees contains no details about the threats, though they describe them as serious and specific.

“We are gravely concerned, in particular, that Congress appears to be the target of a concerted foreign interference campaign, which seeks to launder and amplify disinformation in order to influence congressional activity, public debate, and the presidential election in November,” wrote Pelosi, Schumer, House Intelligence Committee Chairman Adam Schiff, D-Calif., and Virginia Sen. Mark Warner, the top Democrat on the Senate Intelligence panel.

A congressional official familiar with the letter said that there was a classified addendum “to ensure a clear and unambiguous record of the counterintelligence threats of concern.” The person, who was granted anonymity to describe the confidential letter, said the addendum largely draws from the executive branch’s own reporting and analysis.

The FBI said it had received the letter but declined further comment.

While the Democrats’ exact concerns were unclear, there have been worries since Trump was elected that Russia’s efforts to sow American chaos are ongoing in the 2020 election. The 2016 effort included hacking of Democratic email accounts during the campaign by Russian military intelligence officials and the subsequent disclosure by WikiLeaks.

Intelligence authorities said that hack-and-leak operation was aimed at helping Trump’s presidential campaign and harming that of his Democratic opponent, Hillary Clinton. Russians also used a covert social media campaign to spread misinformation and posts aimed at dividing American public opinion. In the end, former special counsel Robert Mueller charged 25 Russian nationals for their roles in foreign interference and influence during the campaign.

Democrats, including members of the Senate intelligence panel, have voiced concerns that an ongoing Republican probe into Biden’s son, Hunter Biden, and his work in Ukraine would amplify Russian disinformation. That probe is being led by Senate Homeland Security and Governmental Affairs Chairman Ron Johnson, R-Wis.

In a statement, Johnson spokesman Austin Altenburg said the committee’s staff had already requested and received a briefing on Russian disinformation and Johnson had requested an additional briefing at the member level.

While Biden criticized Trump for not acting more aggressively against Russia, his pledge that his intelligence community would report “publicly and in a timely manner” on any foreign efforts to interfere in an American election also appeared to be a course correction of sorts to the approach of the Obama administration, which waited months after Russian hacks of Democratic email accounts in 2016 to officially attribute them to Moscow.

Officials did so only after a rigorous internal debate over what they should say. Even when the administration did issue its October 2016 statement blaming Russia for the hacks, it did not mention Putin by name nor an ongoing effort to determine whether the Kremlin’s election interference efforts were being coordinated with the Trump campaign.

Hillary Clinton, who lost the 2016 presidential election to Trump, said Monday she was “glad” Biden was speaking up. “It’s very clear that Russia succeeded,” she told MSNBC. “They believed that they were able to influence the minds and even the votes of Americans, so why would they stop?”