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”.
The president’s own intelligence chief publicly broke with him.
Russia is responsible for “ongoing, pervasive attempts” to undermine US democracy, Director of National Intelligence Dan Coats said in a statement.
Mr Putin denied the claim.
On Monday the US and Russian presidents held nearly two hours of one-on-one talks without their advisers in the Finnish capital Helsinki .