"Impeachment is unpredictable. Who knows how it will unfold knows and what's going to be revealed," Julian Zelizer, a professor of history and public affairs at Princeton University, told ABC News.
"The candidates are going to have to be really nimble. Rather than a static debate on different key issue areas, now they're going to be responding to the news as new things come up," he added.
At the crux of the inquiry is a July phone call between President Trump and Ukrainian President Volodymyr Zelenskiy, in which Trump encouraged his Ukrainian counterpart to work with his personal attorney Rudy Giuliani and U.S. Attorney General William Barr to investigate former Vice President Joe Biden and his son, Hunter. The call set in motion a fast-moving series of events that included a dramatic step forward in Democrats' march toward impeachment.
The politics of impeachment
The move comes as the 19 Democrats running to oust Trump are preparing for the critical run-up to the first votes being cast in Iowa’s early February caucuses in less than five months.
"There are always risks," said Amanda Renteria, a Democratic strategist who worked as the national political director for Hillary Clinton’s 2016 presidential campaign. "But I'm not sure that Democrats had another choice."
Since the start of the 2020 cycle, the Democrats have labored to push the presidential campaign beyond the political drama consuming Washington -- grounding their stump speeches in efforts to reform the health care system, address income inequality, tackle the threats of gun violence and climate change, and most of all, defeat Trump.
The impact of Trump’s controversies on the race is still uncertain, especially given the uncharted political terrain that an impeachment inquiry into this president presents, according to political experts who have studied the historical significance of the three previous instances of impeachment -- in 1868, in 1974, and in 1998.
"Those are all very different moments in American political history," said William Howell, chair of the University of Chicago’s political science department and the director of the Project on Political Reform. "The facts differ in all three of those moments."
"Will it fundamentally alter the terms of the race? I suspect not," he said of the ongoing inquiry. "I still think there will be plenty of discussions about health policy, immigration policy, and what we should do about education debt, etc. But the other concerns about character and motivation are likely to be accentuated."
A chance for Democrats to go on offense
Earlier this year, following the release of former special counsel Robert Mueller’s report on Russian interference in the 2016 presidential election, a steady stream of presidential hopefuls called for impeachment proceedings to begin. But even after Mueller’s testimony on Capitol Hill, partisan differences on impeachment remained as nearly half of Americans showed little movement in an ABC News/Ipsos poll in July.
Renteria asserted that while some Democrats might pitch "bringing the country together," that message might be out of step with this moment.
"We were stronger together," she said, referencing Clinton’s 2016 campaign slogan. "But the intensity around divisiveness was the conversation everybody wanted to have. It feels like if you're not in the ring, figuring out or guiding what side you need to be on, that you're out of the ring."
As Democrats are likely to be met by the challenge of navigating impeachment on the campaign trail, a top Democratic adviser suggested it is "too early to tell" if this will be a campaign issue or affect the 2020 contest.
But the adviser noted that launching an impeachment inquiry feels different than the release of the Mueller report or his testimony because the investigation "was hard for an ordinary person to sort of understand the reason why this is important to them."
"This is just such an easy thing for people to grasp ... I can explain this in 15 seconds. The Mueller investigation took 20 minutes," the adviser said.
Others also caution that Democrats need to keep their impeachment inquiry narrow in scope to maximize its impact and persuade the American public.
"[Democrats] have got to zero in on this Ukrainian matter ... as the clearest example of the president of the United States abusing his power for his own interests by pressing a foreign leader to investigate an American political rival of his," said Doug Thornell, a Democratic strategist. "These actions jeopardize America’s national security ... I think Democrats have got to be disciplined in adhering to that message and not get distracted by the shiny objects Trump will try to hurl at them."
Pitfalls and opportunities
Despite the specter of an impeachment inquiry looming over the White House, the Trump campaign is already executing their counter strategy -- firing off a string of fundraising emails, launching dozens of new Facebook ads in support of the president, and releasing a slickly-produced video decrying Democrats for being "solely focused" on impeaching the president -- which Trump himself tweeted out.
"We’ve had that ready for weeks in case the Democrats were that dumb. And they were," Trump communications director Tim Murtaugh told ABC News.
The campaign raised a total of $5 million in the 24 hours since Pelosi’s announcement, according to Trump campaign manager Brad Parscale.
While an impeachment inquiry does offer Democrats a chance to speak more forcefully to their argument about why Trump should no longer occupy the Oval Office, it also could present complications for a candidate like Biden and hamper his ability to convey a central message of his campaign: his electability in a matchup against Trump.
"I do think that this could hurt Biden. Even if totally false, it muddies the water and brings back memories of the Clinton scandals. Some Democrats might end up seeking a candidate with an unquestioned record," Zelizer said.
For Democratic candidates that are not the direct target of Trump’s claims, an impeachment inquiry could present a promising opportunity to frame the president as a national security liability.
"I think it's definitely going to free up all of the candidates to talk more forcefully, not only about impeachment as a process, but to be even stronger about their views on the president misusing his power and the president being a danger to the country, and not just a bad president," Zelizer said.
Some of the contenders' "brands" may allow them to lean into this moment -- from Biden going directly at Trump in his campaign roll out, to Massachusetts Sen. Elizabeth Warren’s "slaying the dragon" approach to taking on the establishment, to California Sen. Kamala Harris’ career as a prosecutor, Renteria said.
But to effectively run a campaign against the uncertain nature of impeachment, according to Democratic Party strategists, the candidates must prepare "for the unknown" and need to "be flexible."
"This is the part on Democrats … it's going to require a lot of education," Renteria said. "When health care passed, one of the main lessons is that we never marketed what was happening."
Regardless of the strategy or fallout, the 2020 election is shaping up to be one of the most tumultuous campaigns in recent history.
"Trump is intense," Renteria said. "He will get everyone out to vote … we should all be prepared for an incredibly intense year."
ABC News’ Will Steakin contributed to this report.