President Donald Trump is doubling down on a new explanation of his July comments to Ukrainian President Volodymyr Zelenskiy suggesting Ukraine investigate former Vice President Joe Biden , emphasizing that it is his duty as president to investigate corruption regardless of politics even though Biden is Trump's political rival.

The defense seeks to cast as proper the very behavior that is at the heart of an impeachment inquiry into Trump.

"This is about corruption. This is not about politics," Trump told reporters at the White House Friday. "I have an obligation to look into corruption."

Trump suggested to Zelenskiy on a private call that Ukraine launch an investigation into Biden and his son Hunter. The suggestion came soon after Trump froze military aid to Ukraine and it has since been revealed that State Department officials and Trump's personal lawyer, Rudy Giuliani, have been involved in the push to have Ukraine investigate Biden.

Biden is currently the frontrunner in the Democratic race for the presidential nomination and lawmakers are looking into whether Trump used his position to solicit foreign interference in a U.S. election.

Trump has decried the investigation as a hoax and repeated debunked claims that Biden and his son were involved in corruption while Biden was vice president. Trump's new refrain that his suggestion was apolitical and that he was fulfilling his duty comes after he said publicly Thursday that both Ukraine and China should look into the Bidens .

Asked by a reporter Friday if he has ever asked a foreign leader to investigate anyone that was not a political opponent of him, Trump replied, "You know, we would have to look."

While many Republican lawmakers have either defended Trump's actions or said they do not rise to the level of impeachment, some have broken rank to publicly raise concern.

Sen. Mitt Romney, a Republican from Utah, made Friday what may be the strongest condemnation of Trump's actions by a Republican yet.

Cartoons on President Donald Trump View All 948 Images

"When the only American citizen President Trump singles out for China's investigation is his political opponent in the midst of the Democratic nomination process, it strains credulity to suggest that it is anything other than politically motivated," Romney tweeted shortly after Trump's comments to reporters. "By all appearances, the President's brazen and unprecedented appeal to China and to Ukraine to investigate Joe Biden is wrong and appalling."

Trump is also reportedly planning to send a letter to House Speaker Nancy Pelosi refusing to cooperate with subpoenas issued as part of the inquiry unless the full House votes to authorize the impeachment inquiry.

Trump confirmed Friday that the White House will be sending a letter to Pelosi but did not say whether the White House will comply with subpoenas issued by the House.

"That's up to the lawyers," Trump said.

Pelosi has said there is no rule or precedent that requires a full House vote to begin an impeachment inquiry.