Republican senators defended the procedure for President Trump’s impeachment trial Wednesday, saying the House Democrats’ case is unfinished, arguing it’s wrong to ask the Senate to clean up their work.

The move came before the senators gathered to hear opening arguments form the House impeachment managers but after a 13-hour day Tuesday over the trial rules and procedures where Democrats had demanded more opportunities to subpoena evidence.

Sen. Lindsey Graham, South Carolina Republican, said they heard arguments from the House impeachment managers for hours, demanding more witnesses and documents that they didn’t seek during the House impeachment inquiry, recognizing the president would go to court and prolong the process close to the 2020 elections.

But Mr. Graham, chairman of the Senate Judiciary Committee, said it’s the president’s constitutional right to assert executive privilege.

“They didn’t want the court process to stop the impeachment train,” Mr. Graham said of the House Democrats.

Sen. Mike Braun, Indiana Republican, compared the House’s case to a “half-prepared term paper.”

“We are talking about impeaching a president before inauguration — and you’re doing it up through what we are seeing now. That is the context playing out,” Mr. Braun said. “It’s going to vindicate the president.”

Meanwhile, Democrats blasted Republicans for not allowing parties to call witnesses Tuesday, saying they even rejected an amendment that would have allowed Chief Justice John G. Roberts Jr. to decide the relevancy of witnesses.

“The bottom line is this — the very first thing the American people saw when they tuned into the impeachment trial of President Trump was Republican senators voting against having a fair trial with relevant evidence,” said Senate Minority Leader Charles E. Schumer, New York Democrat.

Last month, the House impeached Mr. Trump on two articles — abuse of power and obstruction of Congress.