WASHINGTON — House Intelligence Committee Chairman Adam Schiff said Sunday that Democrats will investigate a range of issues surrounding President Donald Trump’s alleged pressuring of Ukraine in their impeachment inquiry, but insisted that the focus will remain on what he called the "fundamental breach of the president's oath of office."

Schiff's committee is leading the investigation of whistleblower allegations that Trump tried to leverage his office to pressure Ukraine into investigating a top political opponent, former Vice President Joe Biden. Appearing on "Meet the Press" Sunday, Schiff said that Trump's conduct was so "egregious" that the House had no choice but to begin an impeachment inquiry against him.

"The gravamen of the offense here is the president using the power of his office to coerce a foreign nation into helping his presidential campaign to once again interfere in our election, and at the same time withholding foreign aid that country so desperately needs to fight off who? The Russians," he said.

“The situation demands that we move forward with the inquiry.”

The battle over impeachment came to a head last week after Congress compelled the administration to turn over a whistleblower complaint alleging Trump may have been “pressuring a foreign country to investigate one of the President’s main domestic political rivals" around the time the White House froze military aid to Ukraine.

After a flurry of news stories shed some light on the complaint, the White House ultimately released a copy of the complaint, as well as a memo summarizing the July call between Trump and Ukrainian President Volodymyr Zelenskiy, which is at the center of the complaint.

Those notes show Trump asked the Ukrainian president during a July phone call to investigate both allegations of interference in the 2016 American presidential election as well as whether Biden or his son, who worked for a Ukrainian gas company during his father’s tenure, committed wrongdoing.

And the memo also revealed that Rudy Giuliani, Trump's personal attorney, had been making overtures to the Ukrainian government as well.

Schiff told "Meet the Press" that the House is working "urgently" to investigate all facets of the issue — including whether the president committed an impeachable offense by raising the Biden issue, whether Giuliani's overtures were proper, and whether the White House was using the frozen aid as leverage. The chairman didn't commit to a timeline, or to whether he'd call on Giuliani to testify.

But he warned that while he's expecting the White House to "fight us tooth and nail," any attempt to stonewall Congress will only amount to more evidence of possible obstruction.

"The president can't have it both ways — he can't both prevent us from getting evidence on these serious underlying crimes, or potential crimes, this serious breach of his oath of office, and at the same time obstruct our investigation," he said.

“Even as he tries to weaken our ability to get facts on one, he’s going to strengthen the facts on the other."

Schiff’s high-profile position has drawn the ire of Trump, who repeatedly berated the congressman on Twitter over the weekend.

Trump called for Schiff to “resign and be investigated” over comments the lawmaker made at a congressional hearing this week where he said he was re-enacting a parody of Trump’s call with the Ukrainian president.

Trump has repeatedly defended his conduct as proper — confirming key details, including the decision to withhold aid, but arguing that all his actions were aimed at encouraging Ukraine to root out corruption at large. That, he's argued, is good for both American interests and the international community, and has nothing to do with Biden.

But he has frequently repeated his calls for an investigation into Biden in recent days, despite a lack of evidence that Biden or his son did anything wrong. While Biden did pressure Ukraine to fire a prosecutor that was, among other things, investigating the company linked to his son, he did so on behalf of a bipartisan and multinational effort unrelated to that investigation.

House Minority Whip Steve Scalise, R-La., defended Trump's conduct and his handling of the call with Zelenskiy during an interview on "Meet the Press," repeating Trump's call for an investigation into Biden and framing the president's efforts as aimed at investigating 2016 election interference primarily.

"President Trump was looking into the 2016 collusion and interference that Russia had when Barack Obama was president. We all know that happened under Barack Obama's watch," he said.

"I'm glad Trump continues to look into the interference.”

And he framed the investigation as just the latest attempt by Democrats to "go down this road of impeachment, regardless of the facts," pointing to Schiff's interview.

"The chairman said a lot of things, made a lot of baseless accusations. Chairman Schiff was the one who for two years was running around saying he had more than circumstantial evidence that President Trump colluded with Russia on the Mueller investigation. And that turned out not to be true," he said.

“The framers did not put the power of impeachment in the Constitution so that you can stop somebody from being elected who was duly elected."