WASHINGTON — The Senate on Tuesday blocked tax legislation that would have punished U.S. firms that export jobs. But the political symbolism of trying to save American jobs, not passing a bill, was the Democrats' closing argument on the economy in the waning weeks of the congressional elections.Republicans complained that the vote used a serious subject – economic recovery – to score points with voters five weeks before the balloting in which all 435 House seats, 37 Senate seats and the Democratic majority are on the line.