MINNEAPOLIS » Republic Airways Holdings, the 50 percent owner of interisland carrier Mokulele Airlines, said yesterday it will buy struggling Midwest Airlines for $31 million in cash and debt — the second time in two days it has sought to rescue an ailing airline.

On Monday Republic said it would bid $108.8 million to take Frontier Airlines out of bankruptcy.

Republic Chairman and Chief Executive Bryan Bedford said he plans to keep the Midwest name and intends to restore Midwest to the size it was about a year and a half ago. He said he intends to get Midwest flying back to cities it had dropped, citing the recently announced return of flights to Los Angeles and Louisville, Ky., set to begin in August.

Adding Midwest and Frontier are big risks for Republic, and not just because of concerns about the individual carriers.

Until now Indianapolis-based Republic has flown under contract for other big airlines: American, Delta, US Airways, United, and Continental. That meant its main job was controlling costs, not filling seats. But by adding Frontier and Midwest, Republic exposes itself to travel downturns and competition from other airlines.

“;The point is revenue diversity,”; Bedford said of the new additions.

Republic said it would pay $6 million in cash and extend a $25 million note to Midwest owner TPG Capital. The note is convertible to Republic shares at $10 per share. Midwest had owed TPG $31 million, so the sale basically repays TPG for $6 million of that debt and refinances the rest.

Republic said it expects the deal to close in one month to six weeks.

Republic shares spiked after the news hit about a half-hour before markets closed. They rose $1.90, or 47.3 percent, to close at $6.