American Express is getting crushed by its competition

American Express lost its 16-year co-branding agreement with Costco in February, saying the deal no longer made economic sense. And on Monday, Costco announced that it has agreed to a new co-branding agreement with Visa and Citi, starting next April.

In a note Tuesday, Macquarie Research downgraded the credit card company to "Underperform" from "Neutral."

Here's what analyst Vincent Caintic had to say about AmEx:

"AmEx continues to drive strong spend growth outside of Costco (7% y/y domestically in 2015E). However, intense competition makes it difficult for AmEx to maintain share, even with strong industry spends trends in consumer and commercial."

Macquarie is forecasting flat revenue growth for American Express in 2016 due to the loss of the Costco deal. Analysts estimated that Costco earned American Express around $80 billion in billed business annually.

"AXP is no longer a revenue growth story," Caintic wrote as part of Macquarie's bear case. "[It's possible that] AXP becomes more like Discover and other card-loan banks that rely more on the economics of card loans, given a competitive card transaction fee environment."

American Express shares are down nearly 12% year-to-date. Besides the end of the Costco deal, there's been news about the loss of an antitrust lawsuit, and the end of another co-branding agreement with JetBlue.

Caintic wrote that for the share price to return to the level it was before all these headlines, investors need to see 8% revenue growth (compared to the expected 3% currently), and up to 15% growth in earnings per share year-over-year.

Caintic also cautioned investors against being long into the company's March 25 investor day meeting, and said that from current levels, there is only about 7% potential upside to AmEx shares, while downside potential is "significant" at 21%.

Here's how American Express shares have performed in the last few months: