The top Democrat on the Senate Select Committee on Intelligence said President Trump's unwillingness to fully accept that Russia meddled in the 2016 election makes it more difficult to prevent foreign interference in future elections.

"The fact that the president of the United States still refuses to unambiguously acknowledge that the Russians attacked us means that we have no whole of government approach on how we're going to deal with this problem," Sen. Mark Warner, D-Va., told NBC on Sunday.

Warner cited a statistic from the Department of Homeland Security, which has indicated 21 states were hacked, as an example of how Trump's approach has impacted the federal government's efforts to combat future meddling.

The Virginia senator said most secretaries of state have not yet been briefed on whether their states were among those 21 that were targeted.

Trump has been asked multiple times whether he believes Russia interfered in the 2016 election. The president has said he thinks the Russian government played a role, but also said other countries may have meddled in the election.

"I think it could very well have been Russia. I think it could well have been other countries," Trump said earlier this month. "I won't be specific. But I think a lot of people interfere. Nobody really knows. Nobody really knows for sure."

Four intelligence agencies concluded earlier this year the Russian government was attempting to influence the election and working to assist Trump's candidacy.