Radosław Fiedler, professor at Adam Mickiewicz University in Poznań, told the Polish news agency PAP that Trump would govern through executive orders.
But he would also become a "lame duck," he said: not a candidate in the next election and on his way out of politics.
Americans vote on Tuesday for all 435 seats in the House of Representatives and about a third of the Senate.
Governors, state prosecutors and legislators, and local officials are also on the ballot.
National polls give the Democrats a lead of seven to 10 percentage points.
Fiedler put their chances of winning the House at about 90 percent and the Senate at about 60 percent, but cautioned that such figures "are only a snapshot of the public mood on a given day."
Midterms are held halfway through a presidential term, he noted, and the president’s party often loses ground in Congress.
He said Republican candidates were being hurt by high prices, Trump’s "chaotic tariff policy" and an "unpopular war with Iran."
Fiedler also criticised what he called the president’s "oligarchic style of politics," including "the mixing of family business interests with international policy."
The analyst expressed doubts a Democratic win would lead to Trump's impeachment.
The House would have to vote first, then 67 senators, including Republicans, would have to back it in the Senate.
If Trump were removed, Vice-President JD Vance would take over, which might not benefit the Democrats, he said.
He added that Republicans could keep a narrow Senate majority and, even in a 50-50 split, would still have the vice-president’s vote in the chamber.
If the Republicans suffer a heavy defeat, Fiedler predicted they would start looking for a new leader.
"Many things were tolerated about Trump because he had good results. Now he has become a liability," he said.
He suggested the MAGA movement might turn to Vance, who "is already distancing himself from the president, for example over Iran."
Lobbyists and big corporations would also shift their backing to likely winners.
(ał)
Source: PAP