States’ revenue collections are off to a strong start in the current fiscal year, building on gains pegged to the expanding economy and help from the federal tax overhaul.

With most states nearing the midpoint of their fiscal years, which end June 30, at least 19 of them are seeing higher-than-expected general-fund revenue, according to a report from the National Association of State Budget Officers released Thursday. States topping forecasts include Connecticut, Georgia, Pennsylvania and Washington.

Among...