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(CNN) —  

In a Friday interview with Bloomberg, Speaker Nancy Pelosi perfectly diagnosed the biggest danger for Democrats posed by the ongoing impeachment inquiry.

“There is – I should say – a mountain of concerns to be brought up,” Pelosi said. But she went on to add that there is a danger of overwhelming the public with charges and details.

“When does the law of diminished returns set in?” Pelosi, of California, asked rhetorically.

That second part of Pelosi’s quote is absolutely key when it comes to understanding how much political peril lurks in the ongoing impeachment investigation for Democrats. Put simply, that risk is this: The longer the inquiry goes on and the more strands, people and charges are brought into the conversation, the harder it is for people to follow and understand. And the less people understand about the process – and why Democrats believe it to be necessary – the better for President Donald Trump.

Consider this: For months and months, polling showed that a majority of the public opposed impeaching Trump even while, in those very same polls, Trump’s job approval rating hovered at or near 40%. People didn’t think Trump was doing a good job and didn’t want him removed from office.

That polling dynamic changed in the wake of the revelation of the whistleblower complaint regarding the July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky. Suddenly a majority of the country favored Trump’s impeachment and even removal.

While Democratic elected leaders were heartened by that change, it should also serve as a warning sign as to the fickleness of the public on the issue. In a CNN poll conducted late last month, 50% said Trump should be impeached and removed from office while 43% said he should not. While that represents a real change from a few months ago, it’s not as though impeachment is favored by a wide swath of the public. Lose even a little momentum in the eyes of the public, and you’ll soon see data that shows more people prefer keeping Trump in office.

And the surest way to lose that momentum in public perception is to allow the investigation to become an all-encompassing grievance vehicle for Democrats against Trump.

All the proof you need of how these sorts of investigations can explode in a million directions – and to the detriment of the investigators – is Ken Starr and the Starr Report in the mid-to-late 1990s. Starr was initially appointed independent counsel to continue the investigation of the Clintons’ real estate dealings in Arkansas, known collectively as Whitewater.

By the time Starr was done, he had looked into, among other things, rumors of foul play in the death of Vince Foster and Bill Clinton’s affair with a White House intern. House Republicans’ decision to impeach Clinton over his relationship with a White House intern grew out of Starr’s investigation, and led to massive defeats at the ballot box in 1998 as Democrats rallied around Clinton – and against Starr who was effectively portrayed as an out-of-control partisan looking to bring down the former President any way he could.

Sound familiar? Trump is already casting the House investigation as a “witch hunt” driven by Democrats who can’t beat him at the polls and so are trying to end-run the electorate by impeaching him. Given that, the burden on Democrats to KISS (Keep it Simple, Stupid) when it comes to a) what is being alleged b) what is being investigated and c) eventually, why is Trump being impeached is massive.

What these hearings can’t be, Pelosi knows, is a grab-bag of grievance – Russia! Ukraine! Nepotism! Giuliani! – from her caucus. Which, as Pelosi knows, loathes Trump and wants him removed from office like yesterday.

The further Democrats get from this basic fact set – Trump called the Ukrainian President, reminded him that the US does a lot for Ukraine and then asked for him to investigate the Bidens – the more they risk losing public support for their impeachment efforts.

Pelosi knows it. But can she keep her caucus in line?