No one has more riding on Sprint Corp.¡¯s $26.5 billion sale to T-Mobile US Inc. than SoftBank Group and its founder, Masayoshi Son.

Sprint has been going broke for more than a decade, and the wireless carrier¡¯s executives have told regulators the company faces ¡°serious challenges¡± if the takeover by T-Mobile is blocked. With a $19 billion stake in Sprint, SoftBank will bear the brunt of the fallout if the deal collapses.

¡°It would be a test of SoftBank¡¯s willingness to save Sprint,¡± said Amy Yong, a Macquarie analyst. ¡°The ownership stake is so high they¡¯d probably look to save it, but plan B isn¡¯t obvious or very attractive.¡±