WASHINGTON

U.S. President Donald Trump defended Friday his calls on foreign governments to probe former Vice President Joe Biden and his son as an "obligation" as the House of Representatives' impeachment investigation deepens .

"As President I have an obligation to end CORRUPTION, even if that means requesting the help of a foreign country or countries," Trump wrote on Twitter. "This has NOTHING to do with politics or a political campaign against the Bidens. This does have to do with their corruption!"

The House opened its investigation into the president last week, centering on a July 25 telephone call in which Trump repeatedly pressed Ukrainian President Volodymyr Zelensky to investigate the Bidens.

Trump went further Thursday, openly calling for Ukraine to do so in remarks to reporters, and said China should probe the Bidens over uncorroborated claims of corruption.

Trump's call on foreign leaders to probe Biden, who is the leading Democratic candidate heading into next year's presidential polls, has been at the heart of the impeachment investigation.

Soliciting the assistance of a foreign leader to undermine Biden has raised questions of election interference.

Trump first suggested Thursday he has an "absolute right, perhaps even a duty, to investigate" corruption, which he said would include asking foreign government "to help us out."

The remarks drew a swift rebuke from House Intelligence Committee Chairman Adam Schiff, who is leading the impeachment probe.

"It comes down to this," Schiff responded on Twitter. "We’ve cut through the denials. The deflections. The nonsense. Donald Trump believes he can pressure a foreign nation to help him politically. It’s his 'right.'”

The chairwoman of the U.S. Federal Election Commission, Ellen Weintraub, declined to address the specifics of any potential illegality surrounding the president's remarks, but said "the law is pretty clear."

"It is absolutely illegal for anyone to solicit, accept or receive anything of value from a foreign national in connection with any election in the United States," she told MSNBC.

The thing of value does not necessarily have to be financial, she said, and could include information on political rivals.

Trump has denied any wrongdoing, maintaining his call with Zelensky was "perfect" while seeking to undermine the whistleblower whose complaint brought the phone call and other related matters to the public's attention.