JetBlue Airways (JBLU) $3.8 billion bid to acquire Spirit Airlines (SAVE) received support Thursday from two firms that advise major investors how to vote.

​​Institutional Shareholder Services and Glass, Lewis & Co. recommended that Spirit stockholders vote to accept JetBlue's offer when they vote on Oct. 19.

Spirit Chief Executive Ted Christie said Thursday that the companies are continuing to make progress toward completing the transaction.

The two sides initially came to terms in late July, after a lengthy bidding war. Together they will create the fifth-largest U.S. carrier, with about 9% of the market, behind American, United, Delta, and Southwest.

The proposed merger requires U.S. Justice Department approval. DOJ antitrust authorities last year sued to block a proposed merger between American Airlines and JetBlue. A trial is underway in Boston.

U.S. Attorney General Merrick Garland said an American-JetBlue merger would lead to higher fares and fewer flights and routes.

"In an industry where just four airlines control more than 80% of domestic air travel, American Airlines 'alliance' with JetBlue is, in fact, an unprecedented maneuver to further consolidate the industry," Garland said in a statement. "It would result in higher fares, fewer choices, and lower quality service if allowed to continue."

The New York-based JetBlue won its bid for Spirit after a deal between Spirit and Frontier Airlines fell through.

The two companies will continue to operate independently until the agreement is approved by Spirit shareholders and government regulators.