WASHINGTON — As Democrats accelerate their impeachment investigation of President Donald J. Trump, one of the most critical questions around his future is whether Republicans will break ranks.

So far, there is no sign of that from Pennsylvania Sen. Pat Toomey.

In a brief interview Wednesday, he repeated his view that it was “inappropriate” for Trump to press foreign countries to investigate a political rival, Joe Biden, but said that didn’t merit impeachment. The comments from the lone Republican senator in the Philadelphia region suggest he has not been moved by two furious weeks of revelations, including more Trump calls for foreign investigations and the White House’s stonewalling of Democratic investigators.

Toomey noted that no president has been removed from office after having been impeached. “That should tell us something about the gravity of this, the seriousness of this, and why in my view it’s a very high bar,” he said. “There is, I think, latitude in our system to have errors of judgment and inappropriate actions remedied through the political process. It’s called an election.”

Toomey also said the Trump administration should share documents with congressional investigators, but only if House Democrats formally vote to begin an impeachment inquiry and grant some investigative powers to Republicans, including the ability to cross-examine witnesses and issue subpoenas, as has happened in past impeachment inquiries.

The phone interview came after a period that has included additional public calls from Trump for Ukraine and China to investigate Biden; the White House’s declaring that it would not share documents with Congress or allow witnesses to testify before lawmakers; and the revelation of text messages among U.S. diplomats showing that they feared Trump was withholding aid to Ukraine unless he got his desired investigation.

Public opinion polls have shown growing support for an impeachment inquiry, but an almost even split on whether Trump should be removed from office.

Because the Republican-controlled Senate would eventually rule on any formal impeachment charges leveled by the House, political watchers are keeping a close eye on GOP senators for any signs of wavering or distance from Trump.

Toomey is one of the few who has even mildly criticized Trump’s conduct with Ukraine, and he represents a swing state, where pressure could mount faster than for colleagues from more conservative regions.

“It was inappropriate for the president to make a reference to a potential political opponent in the context of an investigation,” Toomey said, referring to Trump’s July phone call with Ukraine’s president, Volodymyr Zelensky.

He had a similar response to Trump’s repeating the call from the White House last week, when he also urged China to dig into Biden and his son, Hunter, though Toomey joined other Republicans in suggesting Trump may have been joking.

“I don’t know whether that was serious or not,” Toomey said, before adding it was “not appropriate, something that I wouldn’t do, but again, not something that would strike me as an impeachable offense.”

As to whether Trump should share documents and allow witnesses to testify in the House inquiry, Toomey argued that Democrats have to change the process first.

“This is a terrible, terrible process, so what we should have is an appropriate process,” Toomey said, accusing House Speaker Nancy Pelosi of delaying a formal floor vote to protect vulnerable lawmakers from going on the record. “If you pursue this properly, even though I might disagree about the merits, then the White House should comply and provide documents.”

Democrats have warned that Trump’s defiance runs afoul of Congress’ broad investigative powers, which Republicans used to probe a wide variety of subjects when they held the House majority.

“Continued efforts to hide the truth of the president’s abuse of power from the American people will be regarded as further evidence of obstruction,” Pelosi said in a statement Tuesday.

With or without a formal House vote to launch an impeachment inquiry, Congress has significant power to investigate the executive branch, said John Hudak, an expert on impeachment from the Brookings Institution.

Members of the House minority were granted subpoena power and other abilities during the impeachment investigations into Presidents Richard Nixon and Bill Clinton. But there are no formal rules for impeachment. In Nixon’s case, lawmakers conducted investigations for months to obtain information before voting to formally open an impeachment inquiry. The fact-gathering on Clinton was mostly done by a special counsel, Kenneth Starr.

“There are no rules that state that the minority should have particular powers within an impeachment inquiry,” Hudak said. “Presidential impeachments are so rare that it is oftentimes done in an ad hoc way.”