Washington — House Democrats on Friday escalated their impeachment inquiry into President Trump, issuing a subpoena to the White House for documents related to the whistleblower complaint. They are also requesting Ukraine-related documents from Vice President Mike Pence.

Mr. Trump maintains he did nothing wrong and is pushing back. He said the White House's cooperation with subpoena requests could hinge on whether House Speaker Nancy Pelosi takes a vote to formally launch an impeachment investigation. The president wants Democrats to go on the record supporting it because the White House thinks it may hurt some of them in vulnerable House districts.

Mr. Trump now says asking foreign governments to investigate former Vice President Joe Biden and his son, Hunter, is not about politics, it's about doing his job.

"I don't care about Biden's campaign but I do care about corruption. We are looking at corruption, we're not looking at politics," Mr. Trump said Friday.

But when pressed if he has ever asked a foreign country to investigate anyone other than a political rival, the president could not recall.

Mr. Trump claims he has an absolute right to ask China and Ukraine to look into what he calls the Biden's "corrupt business deals," even though there is no evidence of any wrongdoing by the Bidens.

He insists he did not withhold $391 million dollars worth of military aid to Ukraine to pressure them to investigate Biden, but text messages from the president's own diplomats provided to Congress tell a different story.

In one message last month, the top U.S. diplomat to Ukraine, Bill Taylor, wrote, "I think it's crazy to withhold security assistance for help with a political campaign."

Gordon Sondland, the Trump-appointed U.S. ambassador to the EU, disagreed, saying, "I believe you are incorrect about President Trump's intentions... No quid pro quos of any kind."

Most congressional Republicans are still backing Mr. Trump, but in a Friday tweet, Utah Senator Mitt Romney called the president's actions seeking foreign help "brazen," "unprecedented" and "appalling."