NASHVILLE — After flirting coyly for months, Sarah Palin this weekend launches an aggressive play to become the leader of the tea party movement, a move with major political upside for the former Alaska governor but also one rife with risk. Her positioning could boost her prospects of securing the 2012 Republican presidential nomination, which she is widely believed to be eyeing. And the tea party is a natural fit for Palin, whose populist anti-Washington rhetoric and working mom persona have made her a movement favorite since its grass-roots activists burst onto the scene last year in opposition to the...