The threat of impeachment is back, and this time investors appear to be paying at least some attention.

“It felt like politics really wasn’t moving markets too much until today,” Art Hogan, chief market strategist for National, told MarketWatch during Tuesday’s session.

Indeed, stocks headed back to session lows in Tuesday afternoon trade after news reports said House Speaker Nancy Pelosi, a California Democrat, would announce a formal impeachment inquiry of President Donald Trump. Stocks ended the day somewhat above those lows but in negative territory, with the S&P 500 index SPX, +1.59% closing 0.8% lower, while the Dow Jones Industrial Average DJIA, +1.33% declined around 140 points, or 0.5%.

Pelosi announced the probe after the close. U.S. stock-index futures pointed slightly lower early Wednesday.

Other factors were also at work on Tuesday, including some downbeat consumer confidence data and a Trump speech at the United Nations that wasn’t exactly warm and fuzzy when it came to U.S.-China trade negotiations.

But stocks did appear to wax and wane in reaction to headlines surrounding Trump’s recent interaction with Ukrainian President Voldomyr Zelensky. At issue is a whistleblower complaint surrounding Trump’s decision to ask his chief of staff to withhold the release of a $391 million aid package to Ukraine more than a week before Trump spoke with Zelensky in a July phone call. In that call, Trump urged the Ukraine president to investigate the business dealings of Democratic presidential candidate Joe Biden’s son, according to The Wall Street Journal.

Stocks had briefly trimmed losses after Trump tweeted that he would release on Wednesday the “complete, fully declassified and unredacted transcript of my phone conversation with President Zelensky of Ukraine…”

Tuesday’s market gyrations, albeit relatively modest, stand in contrast to a market that had remained largely impervious to developments and headlines around Special Counsel Robert Mueller’s investigation into Russian interference in the 2016 election.

So what’s different?

For one, this time around it appears Democrats in the House have momentum toward beginning impeachment proceedings. Second, a formerly robust economic backdrop has given way to jitters about global growth and fears that the U.S. economy is nearing the end of a lengthy expansion. Less confident investors could be more jittery in the face of political headlines than was previously the case.

Also, impeachment proceedings could take center stage in the run-up to the 2020 presidential election, potentially damaging Trump’s re-election bid. Fears of a less business-friendly Democratic administration — amplified by the recent strength of Sen. Elizabeth Warren, who has moved ahead of Biden in some polls — could also be part of the mix, analysts said.

See:Billionaire investor on Elizabeth Warren as president: Market might not even open

Indeed, Greg Valliere, chief U.S. policy strategist at AGF Investments, argued in a Monday note that it was an improved showing by Warren that would be more likely than impeachment to upset market participants.

Before investors get too worked up, however, Hogan said it’s important to remember that initial market reactions to headlines these days are largely driven by computerized trading programs. Once the knee-jerk reaction is out of the way, it will be easier to assess just how concerned investors really are.

Even then, the road map for investors isn’t exactly well illuminated. Stocks saw volatility during the Clinton impeachment process, but continued to climb. In the early 1970s, stocks suffered as the Watergate investigation progressed, culminating with President Richard Nixon resigning in August 1974 to avoid impeachment.

But each incident had plenty of other drivers at work, Hogan noted. In 1998 and early 1999, stocks were in the throes of the tech bubble. By the time Nixon resigned, stocks had been in a downtrend since January 1973 that coincided with the first oil-price shock, which pushed the U.S. into recession (see charts below from Capital Economics).

Capital Economics

Capital Economics

And there are plenty of unknowns to sift through. At present, it appears unlikely a Republican-controlled Senate would convict Trump. And, as Valliere noted, that fact is seen as a top reason why Pelosi has until now been reluctant to press ahead with impeachment, preferring to “defeat Trump at the ballot box instead of giving him an excuse to claim that he’s being hounded by pro-impeachment leftists.”

Impeachment could have an impact, however, on an issue closely followed by investors right now — the U.S.-China trade talks.

“If I’m China and I think the president is going through an impeachment process, I probably sit back before I give in too much at the negotiating table and wait this out,” Hogan said. “That might cast a shadow over one thing that the market is thinking about.”