(Adds Mitt Romney comments)

By Nandita Bose and Lawrence Hurley

WASHINGTON, Sept 22 (Reuters) - If an investigation shows that President Donald Trump pressured Ukraine to investigate former Vice President Joe Biden, the U.S. Congress may have no choice but to pursue impeachment, House Intelligence Committee Chairman Adam Schiff said on Sunday. 

Trump and his allies, including Secretary of State Mike Pompeo, on Sunday defended the president's phone call -- reportedly the subject of a complaint made by an as-yet-unnamed whistleblower -- and escalated their attacks on Biden. 

Schiff had previously shied away from calling for impeachment, but his comments on CNN's "State of the Union" showed his stance had shifted. 

"If the president is essentially withholding military aid at the same time that he is trying to browbeat a foreign leader to do something illicit, to provide dirt on his opponent during a presidential campaign, then that may be the only remedy that is co-equal to the evil that conduct represents," Schiff said. 

Other legislators have called for Democratic leadership to pursue impeachment immediately, but House Speaker Nancy Pelosi, the Democratic leader in the House of Representatives, has so far resisted calls to formally begin the process. 

In a letter to colleagues later on Sunday, Pelosi warned the administration against keeping the details of the whistleblower complaint secret.  The administration has so far resisted sharing the details of the complaint with lawmakers. 

"If the administration persists in blocking this whistleblower from disclosing to Congress a serious possible breach of constitutional duties by the president, they will be entering a grave new chapter of lawlessness which will take us into a whole new stage of investigation," Pelosi wrote. 

Senator Mitt Romney, who has clashed with Trump in the past, sounded a rare note of concern among Trump's fellow Republicans. 

"If the President asked or pressured Ukraine's president to investigate his political rival, either directly or through his personal attorney, it would be troubling in the extreme.  Critical for the facts to come out," he said in a Twitter post. 

Trump's July 25 call with Ukrainian President Volodymyr Zelensky is at the center of the escalating battle in Washington. 

News reports on Friday said Trump repeatedly asked Zelensky to investigate whether Biden misused his position as vice president to threaten to withhold U.S. aid unless a prosecutor who was looking into a gas company in which Biden's son was involved was fired. 

Biden has confirmed he wanted the prosecutor fired but denies it was to help his son.  Biden said the wider U.S. government, the European Union and other international institutions also wanted the prosecutor fired for his alleged failure to pursue major corruption cases. 

Trump, who has denied doing anything improper, told reporters on Sunday he had a great call with Zelensky and that he would have no problem with his personal attorney Rudy Giuliani testifying before Congress. 

"I'm not looking to hurt Biden, but he did a very dishonest thing," Trump said Sunday morning as he left Washington for a rally in Houston. 

Pompeo on Sunday said that Biden should be investigated if evidence shows he improperly intervened to protect his son. 

"I do think if Vice President Biden behaved inappropriately, if he was protecting his son and intervened with the Ukrainian leadership in a way that was corrupt.  I do think we need to get to the bottom of that," he said on ABC's "This Week." 

Democratic Senator Chris Murphy said on NBC's "Meet the Press" that Congress needs to hear from the whistleblower.  If the allegations are sustained, there would be an "open-and-shut" case for impeachment, he said. 

Impeachment proceedings in Congress, which begin in the House, can lead to a president being removed from office, but Democrats would need the support of Republicans, who control the Senate. 

According to Friday's reports, Trump urged Zelensky, a comedian who had just won election, to speak with Giuliani. 

Giuliani, the former New York City mayor, has promoted the allegations against Biden and his son, Hunter, and acknowledged that he pressed for a Ukrainian investigation. 

Biden on Saturday said there should be an investigation into Trump's call, saying it "appears to be an overwhelming abuse of power." 

He said he never spoke to his son about Ukraine. 

(Reporting by Nandita Bose, Sarah N. Lynch and Lawrence Hurley; Editing by Lisa Shumaker, Daniel Wallis and Sonya Hepinstall)

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