All three major U.S. stock indexes ended choppy session essentially flat, with few catalysts and little conviction heading into the Fed's two-day monetary policy meeting.

"(Fed Chairman Jerome) Powell can spark big moves in either direction with his comments and you don’t want to get caught on the wrong side of it," said Peter Tuz, president of Chase Investment Counsel in Charlottesville, Virginia.

The central bank has vowed to remain agile with respect to economic data, which has shown signs that core inflation remains on its meandering descent back toward the Fed's annual 2% target, and suggests the U.S. economy remains on firm footing.

Against this backdrop, growing jitters that a stalemate on Capitol Hill could result in a potential government shutdown had market participants on edge.

Treasury Secretary Janet Yellen on Monday said that while she sees no risk of an economic downtown, she warned that a government shutdown would be "Creating ... a situation that could cause a loss of momentum is something we don't need as a risk at this point."

The week's main event is the Fed's policy meeting, which is expected to culminate in a rate hike pause, leaving the Fed funds target rate unchanged for the second time since March 2022, when the central bank fired its opening salvo in its battle against inflation.

The Federal Open Markets Committee (FOMC) is also due to release its quarterly Summary of Economic Projections, which will include the "dot plot," or a glimpse into participating members' expectations regarding the future path of interest rates.

Financial markets have currently baked in a 99% certainty that the Fed will hold the key rate at 5.25%-5.00% on Wednesday. Beyond that, the trajectory is less certain, with a 69% likelihood of the FOMC holding firm in November, according to CME's FedWatch tool.

"The market would like to see the dot plot come in lower than last time," said Sam Stovall, chief investment strategist of CFRA Research in New York. "It's a case of bad news is good news; most people would say it would be good if the summary economic projections called for economic softening next year," as they gauge the timing of a potential Fed pivot.

On the other hand, the possibility that the softening could mutate into recession remains a top concern.

"Investors are questioning the likelihood of a slowdown versus the hard landing, wondering if things could get worse than forecasters are currently projecting," Stovall added.

According to preliminary data, the S&P 500 gained 3.26 points, or 0.07%, to end at 4,453.58 points, while the Nasdaq Composite gained 1.99 points, or 0.01%, to 13,710.32. The Dow Jones Industrial Average rose 5.79 points, or 0.02%, to 34,624.03.

VF Corp slumped following Piper Sandler's downgrade of the apparel company's shares to "neutral" from "overweight."

British chipmaker Arm Holdings slid after Bernstein initiated coverage with an "underperform" rating just days after its stellar debut.

Paypal Holdings dipped after MoffettNathanson cut its rating to "market perform" from "outperform."