NEW YORK (Reuters) - Major U.S. stock indexes declined sharply on Thursday with investors concerned about the impact President Donald Trump's trade policy may have on companies and the broader economy, while Marvell Technology's ( MRVL ) revenue forecast sparked concerns about spending on artificial intelligence infrastructure.

Below are investor comments about the selloff, which saw the S&P 500 fall below its 200-day moving average for the first time since November 1, 2023.

ART HOGAN, B. RILEY, MARKET STRATEGIST, BOSTON

"The administration seems to be trying to play a ping pong game by announcing something and then pulling it back on tariffs, but this time it's not working. People reacted to Howard Lutnick's attempt to calm the markets with distrust. Clearly, there are signs of a slowdown ahead of any tariffs really digging in and faced with uncertainty, consumers, corporate leaders and investors are all going to freeze and put off longer-term business plans."

"At this point, there's nothing left to give the market a boost now that the excitement of electing a president who was seen as pro-business has worn off."

SAM STOVALL, MARKET STRATEGIST, CFRA, ALLENTOWN, PENNSYLVANIA

"The longer that these tariffs remain in place, the lower that the market is likely to go because of the increasing threat of inflation and recession."

"To make markets feel good, it would have to be a more broad based and more sweeping lift of the tariffs, not just individual sectors, such as automakers."

"Marvell ( MRVL ) earnings is also causing investors to think the AI-trade is slowing down, and it's time to take profits now that we can. The combination of tariffs and technology has been explosive for stock prices."

BILL STERLING, GLOBAL STRATEGIST, GW&K INVESTMENT MANAGEMENT, BOSTON

"A continuation of this on-again, off-again with tariffs particularly with Mexico and Canada (is what is creating uncertainty in markets)."

"The rational economic response to business leaders when there's such a high degree of uncertainty is to sit on their hands and just defer making decisions."

"The other is simply the size of the tariffs. This is way beyond what was experienced in 2018 with the you know so-called China trade war and this could raise inflation, which is what the Fed cares most about, by a full percentage point or a little bit more over the next year."

BRIAN JACOBSEN, CHIEF ECONOMIST, ANNEX WEALTH MANAGEMENT, MENOMONEE FALLS, WISCONSIN

"On-again, off-again tariffs may be worse than just getting the tariffs done with. The uncertainty isn't resolved, it's just prolonged. Businesses will still try to hike prices just in case. Consumers may be more willing to accept price increases because they're afraid of how much higher prices could go. It's not a healthy dynamic. The Fed isn't in a position to run to the rescue."

CAROL SCHLEIF, CHIEF MARKET STRATEGIST, BMO PRIVATE WEALTH, MINNEAPOLIS, MINNESOTA

"A combo of things has come spilling out the worry closet this week - tariffs started it (their actual imposition) and it continues given the flip flopping. Businesses are having a tough time adjusting and the data out recently - including this week - show. Sentiment (business and consumer) is down, inventories way up, job losses are mounting, and commentary from the Fed's beige book all indicate business is having a tough time planning and consumers are concerned."

"Recent tech earnings reports are still making investors question how much longer the data center build-out goes on, even as excitement grows in use cases by more (and smaller) businesses."