WASHINGTON—Democrats have come closer than ever to achieving a long-sought-after minimum wage hike, but opposition is growing against the inclusion of a nationwide wage floor of $15 an hour in the forthcoming coronavirus relief package. Small-business owners in particular will breathe a sigh of relief if the proposal to increase federal minimum wage fails to pass Congress as part of the economic rescue plan. The federal government last raised the minimum wage in 2009, to $7.25. President Joe Biden’s relief plan includes a proposal to increase the minimum wage to $15 an hour over five years, with increases of nearly $1.50 each year. Sens. Kyrsten Sinema (D-Ariz.) and Joe Manchin (D-W.Va.) have already expressed concerns about including the provision in the $1.9 trillion relief package. “The minimum wage provision is not appropriate for the reconciliation process. It is not a budget item. And it shouldn’t be in there,” Sinema told Politico on Feb. …