McDonald’s Corp. 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 $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 on Thursday.

The restaurant chain’s shares (MCD) 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 $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 $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.

“I have zero doubt that other major corporations will follow suit,” Sanders said in an interview with Bloomberg News on Tuesday. He said Walmart Inc., Target Corp., the fast-food industry, and airlines in particular need to raise pay.

McDonald’s (MCD) didn’t immediately reply to a request for comment.

While restaurants are facing pressure to raise wages across the U.S., they’re also dealing with a tight labor market that’s forcing companies and franchisees to hike pay to find enough hourly employees. Olive Garden-owner Darden Restaurants Inc. last month called it a “war for talent.”