Trump reportedly berated Nielsen at a White House meeting earlier this year and some reports suggested she might resign.
Nielsen, however, said "it would be foolish" to think Russian Federation is not still interfering with the US electoral system.
Nielsen said the practice was aimed at deterring child trafficking, and that Congress needed to change immigration laws so that children and parents could be detained together.
"I don't think there's any doubt that they did it, and I think we should all be prepared given that capability and will - that they'll do it again".
"I agree with the intel community's assessment full stop - any attack on democracy, which is what that was, whether it is successful or it is unsuccessful, is unacceptable", Nielsen said.
Nielsen's muddled response on Russian election interference comes during a week in which Trump has been accused of siding with Putin, who has denied the Kremlin meddled in the election, over the United States intelligence community.
Tributes to the 298 people flying flight MH17 tragic. The Airport Of Amsterdam. Source
She said there is no question in the intelligence community or at the Department of Homeland Security she heads that Russian Federation attempted to infiltrate and interfere with the US electoral system.
Following the violent rally a year ago, Trump made statements explaining that there "were fine people on both sides", at the "Unite the Right" white nationalist rally. Her solutions included "partnership", "counter-messaging", and finding a "different way to communicate their message across in a non-violent way".
He said the hearings would be part of a response to election interference by Russian Federation in the upcoming November elections.
"We did not see other nation states involved in the election system meddling", She said. "Because he talked about bringing the US-Russia relationship back to normal". "I think what we've seen on the foreign influence side is they were attempting to intervene and cause chaos on both sides, right, whether it was in Charlottesville where we saw them on both sides - whether it's in Syria - both sides".
President Donald Trump this week has struggled to quiet an uproar over his failure to confront Russian President Vladimir Putin over Moscow's meddling when the two leaders met in Helsinki on Monday. "And what the role of DHS is, is to prepare for them to continue to try to use both foreign influence and direct attacks on infrastructure to effect the results of our election". "We also have white supremacists or other groups who self-profess that their goal or motive is violence".