• JetBlue's latest effort to win over Spirit comes ahead of a shareholder vote on an existing deal between Frontier and Spirit.
• JetBlue increased its reverse break-up free to Spirit shareholders should regulators not approve the deal.
• Frontier sweetened its bid to merge with Spirit late Friday.

JetBlue Airways again increased its offer for Spirit Airlines with a shareholder vote for the discounter's deal to merge with Frontier Airlines just days away.

Frontier sweetened its offer on Friday. Spirit's CEO Ted Christie on Tuesday reiterated the airline's board still found the deal to combine with fellow budget airline Frontier a superior option than going with JetBlue.

Spirit shareholders are set to vote on the Frontier cash-and-stock deal on Thursday; Spirit postponed the vote earlier this month to continue talks with both airlines.

Either combination would create the fifth-largest U.S. carrier. The heated bidding war underscores how both JetBlue and Frontier view Spirit as key to their future growth plans at a time when planes and pilots are in short supply.

Spirit had argued that it didn't think a JetBlue deal would pass muster with regulators, particularly because of its alliance with American Airlines in the Northeast.

"After the Spirit Board's failure to recognize our decisively superior offer, we've discussed our offer directly with Spirit shareholders and are now modifying our proposal in response to shareholders' expressed interest, to include a monthly payment for shareholders, with the certainty of a significant cash premium at closing," JetBlue's CEO Robin Hayes said in a statement.

JetBlue's new offer raises the reverse break-up fee to $400 million from $350 million if regulators don't approve the deal and includes a dividend to Spirit shareholders of $2.50 a share, up from a previous offer of $1.50.

It also includes a "ticking fee," which would pay shareholders 10 cents a share each month from January 2023 through the completion or termination of the deal.

Frontier on Tuesday attacked the new JetBlue offer and dismissed JetBlue's claims that its acquisition of Spirit would lead to lower airfares.

"JetBlue is not telling you the truth," Frontier said in a statement. "A Spirit acquisition by JetBlue would lead to a dead end—a fact that no amount of money, bluster, or misdirection will change. And the only value Spirit stockholders would be likely to receive from JetBlue's proposal is the reverse termination fee, because JetBlue's proposal lacks any realistic likelihood of obtaining regulatory approval."

JetBlue's shares gave up earlier gains to end down 0.3% in a broader market swoon on Tuesday. Spirit's stock closed up more than 1% and Frontier's added 0.6% Tuesday. Shares of those carriers fell sharply Monday.

Frontier on Friday increased the cash portion of its bid by $2 a share to $4.13 and raised its reverse break-up fee to $350 million, matching JetBlue's earlier offer.

"We think we have the most compelling offer for shareholders," Frontier CEO Barry Biffle said in an interview earlier Monday. Biffle spoke from New York, where he is planning to meet with Spirit shareholders this week ahead of the vote on Thursday.