On Monday, Graham asked Clapper during a hearing if he had any concerns about Trump's dealing with Russia. Clapper said he preferred not to comment on the topic because it could be a subject of an existing probe.
Graham added that looking into Trump's tax returns could be another helpful tool to examine the president's business connections with Russia.
US media outlets have repeatedly accused Trump and his associates of allegedly having ties with Russia, which could have contributed to the politician's victory in the recent presidential election.
Russian officials have denied any allegations of meddling in the 2016 US election and the White House has also reaffirmed there was no proof of collusion between Trump's campaign and Russia during the election.
