There has been a barrage of criticism in the US after President Donald Trump defended Russia over claims of interference in the 2016 elections.
At a summit with Russian President Vladimir Putin in Finland, Mr Trump contradicted US intelligence agencies, saying Russia had no reason to meddle.
The top Republican in Congress, House Speaker Paul Ryan, said Mr Trump must see that "Russia is not our ally".
Republican Senator John McCain said it was a "disgraceful performance".
Mr Putin denied the interference claim.
The US and Russian presidents held nearly two hours of closed-door talks in the Finnish capital Helsinki on Monday.
What did President Trump say?
At a news conference after the summit, he was asked if he believed his own intelligence agencies or the Russian president when it came to allegations of meddling in the election.
"President Putin says it's not Russia. I don't see any reason why it would be," he replied.
Mr Trump also blamed poor relations with Russia on past US administrations rather than Russian actions.
US intelligence agencies concluded in 2016 that Russia was behind an effort to tip the scale of the US election against Hillary Clinton, with a state-authorised campaign of cyber attacks and fake news stories planted on social media.
Mr Trump later backtracked, tweeting that he had "great confidence in my intelligence people".
What has US reaction been?
In a strongly-worded statement, Mr Ryan said: "There is no moral equivalence between the United States and Russia, which remains hostile to our most basic values and ideals.
He added that there was "no question" Moscow had interfered in the 2016 election.
Sen McCain, a key member of the Senate Armed Services Committee, said it was a "disgraceful performance" by a US president.
"No prior president has ever abased himself more abjectly before a tyrant," Mr McCain said in a statement.
What did President Putin say?
Speaking at the joint news conference, President Putin offered to allow US investigators to visit Russia to question the officers.
He made it clear that, in return, Russia would want similar access to people in the US it suspects of criminal activity.
In a later interview with Fox News, Mr Putin said it was "utterly ridiculous" that some people thought Russia could have influenced the US elections.
He said US-Russian relations should not be "held hostage" to an internal political struggle in America.
Mr Putin described the Helsinki meeting as "candid and useful" while Mr Trump said there had been "deeply productive dialogue".
Mr Trump said US-Russia relations had "never been worse" than before they met, but that had now changed.
Relations between Russia and the West were severely strained by Moscow's annexation of Crimea from Ukraine in 2014 and Russia's support for separatists in eastern Ukraine.