LUBBOCK, Texas (Reuters) - Texas Attorney General Ken Paxton is expected to fend off a challenge on Tuesday from fellow Republican George P. Bush in a primary run-off election that has pitted a Trump ally against the scion of two former U.S. presidents. Paxton has consistently led in polling in his re-election bid for a third term, though surveys have tightened in recent weeks. He beat out Bush, the state's land commissioner, for former President Donald Trump's endorsement, despite a longstanding securities fraud indictment and other legal woes. The Bush family name was once all-powerful in Texas politics. George P....