"Frankly, I think there's enough people in the Senate who are concerned that what he's doing is robbing from the military and the [Department of Defense] to go and build this wall," Duckworth told "This Week" co-anchor Martha Raddatz.
But Duckworth added she isn't certain there are enough votes in Congress to override an expected presidential veto of such a resolution.
Rep. Jim Jordan, R-Ohio, a backer of the emergency declaration, said in a separate interview on "This Week" Sunday that he agrees with Duckworth that an attempt to override a presidential veto would fall short of the necessary two-thirds support in Congress.
"I think there are plenty of votes in the House to make sure that there’s no override of the president's veto," Jordan said. "So it's going to be settled in court, we'll have to wait and see."
On Friday, Trump signed a bill to fund the federal government through Sept. 30. The deal, reached by congressional negotiators, includes $1.375 billion for a barrier along the U.S.-Mexico border, a figure that is lower than what Democrats proposed last summer.
In a news conference on Friday at the White House, the president also announced the declaration of a national emergency over what he called a "crisis" at the border.
The White House said that through executive authority, Trump has designated an additional $6.5 billion for border wall construction. Most of that money will be redirected from the Pentagon and about $600 million will come from the Treasury Department.
Earlier on "This Week," California Attorney General Xavier Becerra said that the state will be filing a lawsuit "definitely and imminently" against the federal government's emergency declaration.
Reps. Joaquin Castro, D-Texas, and Alexandria Ocasio-Cortez, D-N.Y., are filing a resolution in the House to terminate the national emergency.
Jordan opposes congressional measures to disapprove of the declaration, and took on fellow Republicans who have expressed concern over the president’s move, including Sen. Marco Rubio, R-Fla., who said in a statement on Thursday that "no crisis justifies violating the Constitution."
"Martha, this is an emergency," Jordan said. "I mean, what are we on now? The fifth caravan? So, I would just ask those senators, how many caravans do we need?"
According to data from U.S. Customs and Border Protection, apprehensions at the border have fallen dramatically over the past two decades despite a recent uptick in asylum claims, mostly from families coming from Central America.
Jordan also said the fact that there is "some money" for a barrier in the spending bill passed last week is evidence that Congress hasn't "rejected" the necessity of a wall.
Duckworth agreed that barriers are appropriate in some places along the border.
"Well I think it's appropriate in certain places, but the wall that the president wants to build is not appropriate, and in fact, it's just a fulfillment of a campaign promise that he hasn’t been able to keep," Duckworth said.
Acting Defense Secretary Patrick Shanahan told reporters on Saturday that he has not yet made a determination whether a border barrier is necessary to support the armed forces, discretion that is left to him in the emergency statute.
"We are following the law, using the rules and we're not bending the rules," Shanahan said.
Senior administration officials have said that low-priority military projects would be affected and delayed, while higher priority spending, such as repairs on military family housing, would remain on track.
Duckworth, a member of the Senate Armed Services Committee and Army veteran, said she disagrees with the administration on that point.
"Let me tell you the kind of things that are on the table to be chopped right now," Duckworth said. "A crash and rescue station in Peoria, Illinois, at an airport that I flew out of as a helicopter pilot for 15 years. That's on the table."
Duckworth added that disaster recovery efforts at bases like Tyndall Air Force base in Florida, which was severely damaged during Hurricane Michael in October, could also be impacted.
When asked about the effects on the military, including millions of dollars for projects in Ohio, Jordan said "It's not my decision. It's going to be a decision by those people in the military and the administration and the president of the United States."