A Gallup poll released Wednesday indicated 52% of Americans say President Trump should be impeached and removed from office, up 7 points from June. Democrats and independents were responsible for the increase.

The poll comes as the House impeachment inquiry heated up this week and Trump stepped up his attacks on the investigators.

Among Democrats, 90% are in favor of impeachment while 55% of independents support the action. By contrast, only 6% of Republicans favor impeachment, one point less than June’s results, which came after former special counsel Robert Mueller released his report on Russian interference in the 2016 election.

Forty-six percent of the 1,526 U.S. adults queried Oct. 1-13 said Trump should not be impeached. The survey has a sampling error of 3 percentage points.

Gallup also compared public support for Trump’s impeachment to that for Richard Nixon and Bill Clinton. Support for Trump’s impeachment was well above that for Clinton’s– never higher than 35 percent -- and higher than support for Nixon’s until right before resigned from office. Fifty-eight percent of those queried said they supported Nixon’s impeachment and removal from office in August 1974 as the House prepared to impeach him.

More Democrats support Trump’s impeachment that those who supported Nixon’s but among Republicans support for Nixon’s impeachment was higher than for Trump’s, possibly reflecting a more partisan environment this time around.

“As support for Trump's impeachment and removal from office has grown, so too has approval of the legislative body tasked with the role of putting this process into motion,” Gallup said. “What information comes in the latest round of subpoenas issued by House committees could determine the direction of Americans' support for impeachment.”

The approval rating for Congress rose to 25%, up from 18% last month.

Trump went on the attack on Twitter, calling the impeachment inquiry a witch hunt, complaining about witnesses being deposed behind closed door and insisting the Democrats have no case.

The inquiry was triggered by White House handling of a whistleblower’s report related to a phone call made to Ukraine President Volodymyr Zelensky during which he asked for a “favor,” asking Zelensky to investigate former Vice President Joe Biden and his son.

House Republicans moved Wednesday to force a floor vote on a measure condemning House Intelligence Committee Chairman Adam Schiff for his handling of the impeachment investigation.

“On numerous occasions, Chairman Adam Schiff has used his position to mislead the American people. This censure resolution is about restoring a process that is fair, objective, and fact-based,” House Minority Leader Kevin McCarthy, R-Calif., said. “I call for his censure in the House.”

Republicans also are pushing for a formal House vote on opening an impeachment inquiry, something House Speaker Nancy Pelosi said is not required by the U.S. Constitution. Trump has said he would not cooperate with the investigation without a formal vote.

Schiff said Wednesday five witnesses have backed up allegations Trump misused his power in his July 25 call to Zelensky.

“We’ve made dramatic progress in answering some of the questions surrounding that July phone call,” Schiff said.

He added: “We have learned that call was not in isolation. There was a great deal of preparatory work that was done before the call. There was a lot of follow-up work done after the call.”

Just days before the call, Trump had frozen nearly $400 million in military aid.

Michael McKinley, a 37-year veteran of the State Department, appeared before investigators Wednesday. McKinley was a senior adviser to Secretary of State Mike Pompeo before resigning last week. He told investigators career diplomats were mistreated and their careers wrecked for political reasons. He characterized the ouster of Marie Yovanovitch as ambassador to Ukraine as punitive and wholly unjustified.

Energy Secretary Rick Perry, who has said he urged Trump to make that July 25 call to Zelensky, received a subpoena for records from investigators but refused to say Wednesday whether he would comply.

Gordon Sondland, the U.S. ambassador to the European Union and a Trump donor, was scheduled to testify Thursday. The administration stopped him from appearing last week.