"They've taken away our rights," Donald Trump told reporters Friday.

Highlights Trump said Democrats had enough votes to impeach him

He suggested Speaker Nancy Pelosi hold a House vote

Some Republicans publicly broke ranks with him Friday

President Donald Trump conceded Friday that Democrats had enough votes to impeach him, but he suggested that Speaker Nancy Pelosi hold a House vote to formally begin an inquiry to force a Senate trial on whether to remove him from office.

"They've taken away our rights," Trump told reporters Friday, as he capped a tumultuous week when new revelations about his administration's dealings with Ukraine emerged each day. "They're all in line. Because even though many of them don't want to vote, they have no choice. They have to follow their leadership. And then we'll get it to the Senate, and we're going to win."

But even as Trump boasted of a "very unified" Republican Party that would protect him from conviction, some Republicans publicly broke ranks with him Friday. At least two GOP senators and one former administration official expressed uneasiness with Trump's efforts to encourage foreign governments to investigate former vice president and 2020 Democratic presidential candidate Joe Biden.

Separately, intelligence community inspector general Michael Atkinson met with lawmakers Friday to discuss a whistleblower complaint alleging abuse of power by Trump. Atkinson, a Trump appointee, previously said that the whistleblower "appeared credible" and that the complaint represented an "urgent concern" worthy of Congress' immediate attention.

And documents reviewed by The Washington Post showed that Kurt Volker, the former U.S. special envoy to Ukraine, defended Biden in a statement to Congress that directly undercut Trump's claims of corruption by the former vice president.

"I know him as a man of integrity and dedication to our country," Volker said in his testimony Thursday.

Trump's concession that he would probably be impeached by the House was the latest development in what has become an ad hoc response strategy largely shaped by the president's impulses. Since Democrats announced their inquiry last week, Trump has shown flashes of anger, frustration, aggression, defiance and even indifference.

On Friday, Trump continued to take a combative stance and cast himself as a victim of overzealous Democrats. "We've been treated very unfairly, very different from anybody else," he said.

Trump said he would spell out his complaints in a letter to Pelosi, D-Calif., whom Republicans have increasingly accused of short-circuiting the formal impeachment process by not holding a vote on the House floor to launch an inquiry.

The process-based argument has become a central part of the GOP response to the impeachment debate, with few Republicans publicly defending Trump's behavior or his assertion that he has the "absolute right" to ask foreign governments to investigate his political opponents.

Meanwhile, House Democrats have ramped up their inquiry, interviewing key Trump administration officials and issuing subpoenas as part of their probe of the president's dealings with the Ukrainian government. On Friday, three House committees subpoenaed the White House for documents and wrote a letter to Vice President Mike Pence, demanding that he turn over documents related to his talks with Ukrainian President Volodymyr Zelensky.

The letter called for Pence to deliver documents by Oct. 15 to explain what role he had in the White House's effort to pressure Zelensky to open investigations of Trump's political opponents. Pence met with Zelensky last month in Poland as the White House was withholding nearly $400 million in aid approved for Ukraine.

Pence's office dismissed the request as unserious.

"The Office of the Vice President received the letter after it was released to the media and it has been forwarded to Counsel's Office for a response," Katie Waldman, spokeswoman for the vice president's office, said in a statement. "Given the scope, it does not appear to be a serious request but just another attempt by the Do Nothing Democrats to call attention to their partisan impeachment."

Democrats are investigating whether Trump or others in his administration linked the release of the aid to the president's request that Ukraine investigate Biden and his son Hunter.

During a July phone call with Zelensky, Trump pushed for Ukrainian prosecutors to work with Attorney General William Barr and Trump's personal attorney Rudy Giuliani on an investigation of alleged corruption by the Bidens.

Hunter Biden served for nearly five years on the board of Burisma, Ukraine's largest private gas company, whose owner came under scrutiny by Ukrainian prosecutors for possible abuse of power and unlawful enrichment. Hunter Biden was not accused of any wrongdoing in the investigation. As vice president, Joe Biden pressured Ukraine to fire its top prosecutor, Viktor Shokin, who Biden and other Western officials said was not sufficiently pursuing corruption cases. At the time, the investigation into Burisma was dormant, according to former Ukrainian and U.S. officials.

Republicans have struggled to find a consistent defense of Trump in the wake of the whistleblower's report, which was published last week. The anonymous whistleblower claimed that Trump pushed for the Ukrainian government to investigate his political rival, allegations that have been confirmed as the congressional probe has uncovered text messages, internal documents and sworn testimony from the Trump administration.

"Democrat House members cannot be allowed to hide behind @SpeakerPelosi when it comes to an impeachment inquiry of President @realdonaldTrump," Sen. Lindsey Graham, R-S.C., tweeted Friday. "They should - and must - vote to open an inquiry of impeachment so their CONSTITUENTS, COUNTRY, and HISTORY can evaluate their actions."

House Minority Leader Kevin McCarthy, R-Calif., sent a letter to Pelosi on Thursday requesting that she suspend the impeachment inquiry and ensure that the full House be allowed to vote on whether to proceed. McCarthy said that Pelosi risked creating "a process completely devoid of any merit or legitimacy" if she did not follow specific guidelines to give "the bare minimum rights granted to his predecessors."

In an interview with The Washington Post, Pelosi said holding a vote on the House floor was an option but not a requirement for proceeding with an inquiry.

"There's nothing anyplace that says that we should. However, the people who are most afraid of a vote on the floor are the Republicans," she said. "That's why they're beating their tom toms like they want it, but they don't. They have the most to be concerned about because for some of their members, to say that we shouldn't go forward with this is a bad vote."

Some Republicans have spoken out against Trump's behavior. After the president said Thursday that China should also investigate Biden, Sen. Mitt Romney, R-Utah, said it was "wrong and appalling."

"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, the 2012 Republican presidential nominee, said in a statement Friday.

Sen. Ben Sasse, R-Neb., who is up for reelection next year, distanced himself from Trump's call for a Chinese investigation targeting Biden.

"Americans don't look to Chinese commies for the truth," he said in a statement to the Omaha World-Herald.

Sen. Marco Rubio , R-Fla., did not defend Trump but said he did not take the president's calls for foreign investigation of political rivals seriously.

"I don't think it's a real request," he told reporters Friday.

But the release of several text messages from the State Department late Thursday revealed a concerted effort to push Ukraine to begin the investigations desired by Trump.

The texts show how Volker and other State Department officials coordinated with Zelensky's top aide and Giuliani to leverage a potential summit between Trump and Zelensky on a promise from the Ukrainians to investigate Burisma.

In his statement to Congress, Volker said his efforts to persuade Trump to support Zelensky were undermined by information that Giuliani and others had been feeding the president about corruption. Trump has since linked those corruption allegations to Biden, calling for an investigation.

Texts among Volker, Giuliani and the U.S. ambassador to the European Union, Gordon Sondland, showed that the men coordinated on a draft statement for the Ukrainians in an effort to satisfy Trump's demands. After sending it to Giuliani, who wanted an explicit reference to Burisma and the 2016 election investigation, Volker and Sondland messaged back and forth to work up the text to send back to the Ukrainians, according to Volker. The Ukrainians ultimately did not agree to the statement upon receiving that version.

At least one U.S. official, William "Bill" Taylor, the charges d'affaires in Ukraine, voiced concern about an apparent quid pro quo.

"As I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign," Taylor texted Sondland on Sept. 9, complaining that the Trump administration's decision to withhold congressionally approved military aid to Ukraine had already created a "nightmare scenario."

"The president has been crystal clear no quid pro quo's of any kind," Sondland replied.

On Friday, Trump defended the texts by referring to Sondland's statement. "He said, by the way, there's 'no quid pro quo,' " the president said. "And there isn't."