JetBlue goes hostile in its takeover bid for Spirit Airlines.

JetBlue said Monday that it was beginning a tender offer to acquire Spirit Airlines after Spirit’s board rebuffed an earlier proposal, sticking to its plan to merge with Frontier Airlines.

In a statement, JetBlue said it was offering $30 a share to buy up all of Spirit’s outstanding shares directly from shareholders. That would set the purchase price at more than $3.2 billion.

Spirit stock closed Friday at $16.98 and rose 16 percent in premarket trading on Monday, to $19.70, after the JetBlue offer was announced.

JetBlue said it was willing to increase the offer to $33 a share, the price it initially offered in April, if Spirit cooperated and shared the same information about its business with it as it has with Frontier.

JetBlue said it had filed a “vote no” proxy statement calling for Spirit shareholders to vote against the Frontier merger at an upcoming special meeting. JetBlue said that its all-cash offer was currently worth 60 percent more than Frontier’s cash-and-stock proposal. Frontier’s share price has fallen by about a third since it announced its merger with Spirit, reducing the value of its bid.

Two weeks ago, Spirit’s chairman, Mac Gardner, said that “after a thorough review and extensive dialogue with JetBlue,” his company stood by its plan to merge with Frontier, arguing that it reflected the best interests of long-term shareholders.

Spirit and Frontier, both low-fare airlines, announced a plan to merge in February. JetBlue then stepped in last month with its own offer for Spirit. Either deal would come under U.S. government scrutiny, given the skepticism of regulators appointed by the Biden administration about industry consolidation.

JetBlue already faces an antitrust lawsuit by the Justice Department seeking to nullify an alliance between JetBlue and American Airlines in the Northeast, a deal that one official described last year as a “de facto merger.”

The proposed merger between Spirit and Frontier has also spurred concerns. In March, several progressive lawmakers, including Senators Elizabeth Warren, Democrat of Massachusetts, and Bernie Sanders, independent of Vermont, expressed misgivings, warning that the merger could raise ticket prices and harm customer service.

A Spirit-Frontier merger would combine two budget carriers with strengths on opposite coasts. The competing offer could accelerate JetBlue’s plans to compete with the four big U.S. carriers — American Airlines, Delta Air Lines, United Airlines and Southwest Airlines — which have a combined 66 percent share of the domestic market.

A combined Frontier and Spirit would control over 8 percent of the market, and JetBlue and Spirit together would command more than 10 percent.