"If the Republicans are able to take over, [they] have a much more sort of extreme group of people, then their demands might be unacceptable in any way, shape, or form," Reed said on Monday.
"So we could be in a situation where our domestic turmoil and our domestic arguments are such that the collateral damage, as I said before, is to our support for Ukraine."
At the moment, Republicans and Democrats in Congress have been unified in providing support to Ukraine amid Russia's special military operation, Reed said.
Senate Republicans will have an opportunity to take control of the House and Senate in the US midterm elections on November 8.
All 435 seats in the House of Representatives and 35 of the 100 seats in the Senate will be contested. Democrats currently control both chambers of Congress.