Stocks tumbled Wednesday and major indices notched their biggest declines in months, which some market watchers attributed to ongoing controversies related to President Trump and Russia.

The Dow Jones Industrial Average fell by 373 points, or nearly 2 percent, and the S&P 500 recorded a similar tick down, the largest drops since September for both.

Bank stocks led the decline, suggesting to analysts that downgraded expectations for pro-business legislation and regulation created the downturn.

"If the market is giving some chance to some of the policy changes out there, whether it be tax reform, lower regulations, things of that nature, I think that the odds of those clearly decline with the kind of change we've had in tone" resulting from the latest news about President Trump, said Patrick Kaser, a portfolio manager for Brandywine Global, speaking on CNBC.

The latest news to inject uncertainty into the political sphere was the late-breaking reports Tuesday night that fired FBI Director James Comey had written a memo claiming that Trump had asked him to drop the investigation in Mike Flynn, Trump's dismissed national security adviser.

By Wednesday, some congressional Republicans had indicated that they viewed the report as a potentially serious problem for Trump.

While some members acknowledged that the controversy surrounding Trump could slow legislation, others have maintained that big-ticket items like tax reform remain on track.