McDonald’s Corp. (MCD.N) shares slipped as Senator Bernie Sanders called on the fast-food chain to follow in Amazon.com Inc.’s footsteps and boost its minimum wage.

“If McDonald’s raised the minimum wage to US$15 an hour and respected the constitutional rights of your workers to form a union, it would set an example for the entire fast food industry to follow,” the Vermont Senator wrote in a letter to the company’s chief executive officer, which he posted to his Twitter account.

The restaurant chain’s shares pared earlier gains following the tweet, briefly turning negative for the day before reversing course.

In recent years, the company has faced pressure from the union-backed Fight For US$15 that’s demanded fast-food chains pay more and allow workers to organize. As McDonald’s moves to overhaul its chain with remodeled stores that include more self-order kiosks, there’s been speculation it will cut the need for hourly workers. But McDonald’s recently has said this isn’t true, with more workers actually required. The chain also said in 2015 it would raise pay at U.S. company-owned stores by at least US$1 above the local minimum wage.

Sanders’ letter to CEO Steve Easterbrook follows news this week that Amazon will offer $15 an hour in November to more than 250,000 current employees, along with 100,000 more seasonal workers -- drawing praise from the Democratic lawmaker himself.