WASHINGTON (Reuters) - As televised hearings on whether to impeach U.S. President Donald Trump begin this week, witnesses are preparing for a grilling by lawmakers keen to score political points in front of ranks of TV cameras, watched by a public tuning in that may not know the whole story.

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The testimony from State Department witnesses presents the first opportunity for the public to hear directly from those most closely involved in the events that sparked the impeachment inquiry, and will likely play a key role in building or eroding support.

Democrats are investigating whether Trump pressured Ukraine to tie security aid to probes that might benefit his 2020 re-election bid. Trump has denied any wrongdoing and branded the investigation a hoax.

Jack Quinn, who once served as White House counsel to President Bill Clinton, said the witnesses must maintain equanimity, despite hostile questioning they are likely to face from Republicans like Ohio Representative Jim Jordan, a Trump supporter.

“The important thing is that they not let someone like Jordan get under their skin and cause them to say something that could be interpreted as bias,” he said.

He said witnesses should not act like they are “out on the back porch with your next door neighbor... You don’t need to be chatty. You need to be accurate and precise.”

The two witnesses set to appear on the first day of public hearings on Wednesday - Bill Taylor, the top U.S. diplomat in Ukraine, and deputy assistant secretary of state George Kent - have already answered questions behind closed doors for House committees probing whether U.S. policy in Ukraine was tainted by the president for personal gain.

The testimony will be carried by major broadcast and cable networks and is expected to be viewed by millions, who will watch current and former officials from Trump’s own administration begin to outline a case for his potential removal from office.

It has been 20 years since Americans last witnessed impeachment proceedings, when Republicans brought charges against then-Democratic President Bill Clinton.

CONNECTING THE DOTS

Nick Allard, who worked for former Senator Ted Kennedy on the Judiciary Committee and is now a professor at Brooklyn Law School, said the witnesses should describe their resumes to show they are experienced public servants who have served under both Democratic and Republican administrations.

Allard said they also should be ready to explain how the Trump administration’s actions in Ukraine compare to other experiences in their careers. “Is it anything you’ve seen before? Is it normal, or is it highly abnormal or unprecedented?”

Democrats will want the witnesses to connect the dots to help people understand what happened, why the aid was blocked and by whom, and the role of the president’s lawyer Rudy Giuliani.

There may be cinematic moments, said Allard, recalling how, in 1973, White House former deputy chief of staff Alexander Butterfield told the world that President Richard Nixon had a taping system in the Oval Office, a revelation during the ‘Watergate’ hearings that helped lead to his resignation.

“I remember that vividly,” Allard said. “People’s jaws dropped.

“You could have that kind of moment here.”