By Mark Felsenthal URBANDALE, Iowa, Sept 1 (Reuters) - President Barack Obama tried to bolster his re-election campaign on Saturday with a fierce critique of the Republicans' convention and a plea to supporters to cast their ballots as early as possible. Speaking to a crowd of 10,000 in the battleground state of Iowa, Obama said rival Mitt Romney and his fellow Republicans had offered no new ideas when they held the national spotlight for three days during their convention in Tampa.