WASHINGTON—U.S. economic growth accelerated in the first quarter, fueled by massive government aid to households and businesses, setting the course for what is expected will be the strongest performance this year in nearly four decades. Gross domestic product increased at a 6.4 percent annualized rate last quarter, the Commerce Department said on Thursday in its advance estimate of GDP for the first three months of the year. That was the second-fastest GDP growth pace since the third quarter of 2003 and followed a 4.3 percent rate in the fourth quarter. Economists polled by Reuters had forecast GDP growth increasing at a 6.1 percent pace in the January-March period. The United States’ economy is rebounding more quickly compared to its global rivals, thanks to two additional rounds of COVID-19 relief money from Washington as well as easing anxiety over the pandemic, which has boosted domestic demand and allowed services businesses like …