Has Trump overstepped limits of power? Public hearings begin.

Wednesday's opening marks the fourth time in U.S. history that Congress has launched impeachment proceedings against a sitting president.

First Look

For three years, Donald Trump has unapologetically defied the conventions of the American presidency. On Wednesday, he comes face to face with the limits of his power, confronting an impeachment process enshrined in the Constitution that will play out in public and help shape how the president will be viewed by voters next year and in the history books for generations.

Mr. Trump accepted the Republican nomination, declaring that "I alone can fix" the nation's problems. Once elected, he set about reshaping the presidency, bending and dismantling institutions surrounding the 230-year-old office.

Now a parade of career public servants will raise their hands and swear an oath to the truth, not the presidency, representing an integral part of the system of checks and balances envisioned by the Founding Fathers.

"Trump can do away with the traditions and niceties of the office, but he can't get away from the Constitution," said Douglas Brinkley, presidential historian at Rice University. "During Watergate, many people feared that if a president collapsed, America is broken. But the lesson of Nixon is that the Constitution is durable and the country can handle it."

It's only the fourth time in American history that Congress has launched impeachment proceedings against a sitting president. Two of those – against Andrew Johnson in 1868 and Bill Clinton 130 years later – resulted in their impeachments, or formal charges approved by the House. Both were acquitted by the Senate.

President Richard Nixon resigned in 1974 before the House could vote to impeach him.

The Democrats will try to make the case that the president tried to extort a foreign nation, Ukraine, to investigate a political rival, former Vice President Joe Biden. But even if the House ultimately votes to make Mr. Trump only the third American president to be impeached, few expect the Republican-controlled Senate to eventually remove Mr. Trump from office.

"Even if reelected, it's a dark mark," Mr. Brinkley said. "He does not get off scot-free. There is a penalty you pay."

Mr. Trump enters the crucible of the public hearings largely alone – by his own design.

He has killed the White House daily press briefing, likes to make announcements himself on Twitter, and prefers to get his message out during chaotic jousting sessions with reporters in the Oval Office or as he comes and goes to his presidential helicopter. He has railed against the lack of support from his staff and Republicans on Capitol Hill, insisting that they stop limiting their complaints to the impeachment process and start defending his actions, a request that has unsettled some Republicans trying to get a handle on ever-shifting explanations coming from the White House.

Although a number of the president's advisers believe that impeachment could be a political winner for Mr. Trump on the campaign trail next year, the president has reacted angrily to the probe. He defends his summer phone call with Ukraine's leader, which is at the heart of the inquiry, as "perfect" while deriding the impeachment effort as a conspiracy among Democrats and the "deep state."

Some help is on the way. The White House bolstered its communications team by hiring former Florida Attorney General Pam Bondi and former Treasury spokesman Tony Sayegh. But Ms. Bondi and Mr. Sayegh may not be in place before Wednesday's hearings, owing to paperwork associated with entering White House employment, according to a White House official who spoke on the condition of anonymity to discuss personnel matters.

The Republican National Committee will be lining up supporters to publicly defend the president, including a Thursday conference call for regional reporters with presidential son Eric Trump that is aimed at putting pressure on vulnerable House Democrats. Many of them represent districts that the president won in 2016.

Although Mr. Trump teased Tuesday that he will soon release the transcript of his April phone call with Ukrainian President Volodymyr Zelenskiy, White House officials are not confirming that any such release is forthcoming. That first call to Mr. Zelenskiy is widely known to have been largely a congratulatory conversation after Mr. Zelenskiy's election. It was the rough transcript of Mr. Trump's second call with Mr. Zelenskiy, in July, that prompted a whistleblower's complaint.

Releasing a transcript of the first call could be an attempt by the White House to distract from the congressional hearings, though the impeachment inquiry has moved well beyond the phone calls into broader attempts by the president and his allies to prod Ukraine to investigate Democrats by using U.S. military aid as leverage.

Mr. Trump has his own version of counterprogramming ready to go up against the hearings. He is scheduled to hold a noon meeting Wednesday with Turkey's Recep Tayyip Erdogan and hold a joint afternoon news conference with the Turkish leader. Their meeting comes just weeks after Mr. Trump's decision to pull most U.S. forces out of Syria led to a violent Turkish invasion.

In the morning, Mr. Trump is expected to watch the impeachment proceedings from the White House residence and on a TV just off the Oval Office.

The president's supporters, meanwhile, have been working to discredit the proceedings by finding fault with the way the process has played out and the cast of witnesses who have come forward to testify.

"At its core, this is an impeachment push by career bureaucrats to undermine President Trump's 'America First' foreign policy and politically minded Democrats who want to kneecap him ahead of the 2020 election," said Jason Miller, senior adviser to Trump's 2016 campaign. "If Republicans stick together, Trump will not just survive this, he will defeat the impeachment hoax and be re-elected. It's merely the latest episode in a pattern of Democrats and unelected bureaucrats trying to undermine the presidency."

The timetable for the impeachment proceedings is not firm. But a trial in the Senate, were it to occur, could stretch until the first presidential votes are cast in February's Iowa caucus. The final stakes could rest with the voters next year.

"Trump is now up against the Constitution, but he's not the only thing on trial: So are we the people, as the preamble described us so long ago," said presidential historian Jon Meacham of Vanderbilt University. "Impeachment is a political, not a legal, process, and those with a political stake in this presidency – which is to say, his supporters at large and in the House and the Senate – need to decide which is more important: the efficacy of checks and balances or the continued reign of a president who seems to take pleasure in flouting those checks and balances."

This story was reported by The Associated Press. Zeke Miller, Jill Colvin, and Laurie Kellman contributed to this report.