The top US counterintelligence official on Friday warned that Russia, China, and Iran will all try to interfere in the 2020 presidential election, with Russia already trying to undercut Democratic candidate Joe Biden.

In an unusual public statement, William Evanina, director of the National Counterintelligence and Security Center, said the three countries were using online disinformation and other means to try to influence voters, stir up disorder, and undermine American voters’ confidence in the democratic process.

President Donald Trump, asked at a news conference in New Jersey how he would respond to interference in the Nov 3 vote, said: “We’re going to...