ATHENS, Greece (AP) — Greece's outgoing prime minister, Alexis Tsipras, is banking on his popularity to win a national election next month and strengthen his grip on power after purging his radical left Syriza party of dissenters.But as the political jostling heats up ahead of the Sept. 20 vote, it appears increasingly likely that Tsipras will have to form a new, more unwieldy coalition government — possibly with as many as three other parties.Tsipras called the election last week to rid his party of rebels who had refused to back the bailout deal he agreed upon with Greece's international creditors.