President Donald Trump on Monday intensified his attempts to justify a phone call with Ukraine’s president in which he reportedly pushed for an investigation into one of his chief political rivals, maintaining that he‘s merely concerned about stamping out corruption.

He once again dismissed the accusations that he was using his position of power to pressure a foreign government into investigating former Vice President Joe Biden and his son as a “witch hunt.” For the first time, however, Trump appeared to connect the idea to the issue of foreign aid.

“If we're supporting a country, we want to make sure that country is honest,” Trump told reporters after arriving for the U.N. General Assembly. “If you don't talk about corruption, why would you give money to a country that you think is corrupt? One of the reasons the new president got elected is he was going to stop corruption. So it's very important that on occasion you speak to somebody about corruption. Very important.”

Trump’s defense comes as reports of the call have reinvigorated a push for his impeachment by Democrats who say the president’s decision to urge an investigation into Biden was out of bounds. Also at issue with critics of the president is $250 million in foreign aid meant for Ukraine that the White House mysteriously withheld before releasing under bipartisan pressure early this month. There is no evidence that Trump directly tied the release of foreign aid to an investigation into Biden and his son.

Trump also said he was “not at all” taking seriously the threat of impeachment in light of the latest revelations and again insisted that everything he said to President Volodymyr Zelensky during a late July phone call had been above board.

“We had a perfect phone call with the president of Ukraine — everybody knows it,” he said, casting the issue as a repeat of the Russia investigation, which he alleged was meant to derail his presidency. “Here we go again. They failed with Russia, they failed with recession, they failed with everything. And now they’re bringing this up.”

Trump’s story has evolved somewhat in the days since House Democrats’ struggle to get an anonymous whistleblower complaint spilled out into the public and details about the contents of the complaint began leaking out into the press. Trump first danced around the subject of whether he brought up Biden, whom Trump has accused of using his vice presidential perch to fire a Ukranian prosecutor investigating the company connected to his son, Hunter.

But on Sunday, Trump appeared to acknowledge for the first time that he raised the subject in his call with the newly elected leader.

“The conversation I had was largely congratulatory. It was largely corruption — all of the corruption taking place,” Trump told reporters while departing the White House. “It was largely the fact that we don’t want our people, like Vice President Biden and his son, creating to the corruption already in the Ukraine.”

Biden, for his part, has forcefully denied any wrongdoing and ripped Trump and his allies for suggesting otherwise. Over the weekend in Iowa, Biden called reports of Trump’s outreach an apparent “overwhelming abuse of power.” He's also called for Trump to release the transcript of his phone call with Zelensky, something the president has suggested was a possibility.

The Wall Street Journal reported on Friday that Trump had pressured Zelensky around eight times to investigate Biden and his son during the call, telling the Ukranian president to work with his personal attorney, Rudy Giuliani, who has also been pushing the issue in the press.

Trump on Monday also cast doubt on the whistleblower whose complaint set off the scramble to learn more about Trump’s phone call with Zelensky. Though the intelligence community’s internal watchdog, a Trump appointee, deemed the complaint was urgent and credible enough to meet the statutory threshold requiring it be turned over to Congress, Trump and his allies have painted the complainant as a partisan adversary of the president.

“These people are stone cold Crooked,” Trump said of his detractors in a tweet, before accusing the whistleblower of getting unspecified facts wrong.

“Also, who is this so-called ‘whistleblower’ who doesn’t know the correct facts. Is he on our Country’s side. Where does he come from. Is this all about Schiff & the Democrats again after years of being wrong?” he asked.

Trump also suggested that the drive to investigate the newest allegations was borne out of Democratic disdain that special counsel Robert Mueller’s investigation was insufficiently politically damning.

“If you look at what Biden did, Biden did what they would like to have me do,” Trump claimed Monday to reporters. “Except for one problem: I didn't do it.”