Republicans have run the gamut in their defense of President Trump’s interactions with Ukraine in recent days, as the public hearings in the House impeachment inquiry loom: There was no quid pro quo. Or if there was, it wasn’t illegal or even improper. And because Ukraine ultimately received aid and its president met Mr. Trump without opening investigations, no favors were traded.

The impeachment inquiry centers on reports that Mr. Trump withheld aid to Ukraine while he was pressing the country to investigate Democratic presidential candidate Joe Biden and his son, Hunter.

Mr. Trump has made it clear he wants GOP lawmakers to defend him more on substance, not merely complain about the House process. “I’d rather go into the details of the case,” he told reporters last week.

Many Republicans, however—Senate Majority Leader Mitch McConnell chief among them—are sticking with criticism of how Democrats have conducted the inquiry and ducking questions on the merits of the case.

At a closed-door lunch with GOP senators this week, Vice President Mike Pence commended House Republicans for sticking together in opposition to the vote affirming the impeachment inquiry, according to a person familiar with the matter. Mr. Pence encouraged the senators to review the transcript of the July call between Mr. Trump and Ukrainian President Volodymyr Zelensky and stressed that the administration did ultimately release the aid to Ukraine, the person said. Mr. Pence also dismissed the House impeachment process as a sham.