With just over three weeks to go until Election Day, more and more red states and congressional districts appear in reach for the Democrats, as they work to win back House and Senate majorities from the Republicans in the midterms. But a massive Republican spending advantage threatens to complicate those ambitions.Republican campaigns, committees and aligned groups have outspent their Democratic counterparts by $400 million so far this election cycle, according to AdImpact, a leading political advertising tracking firm.The disparity is forcing Democrats to make difficult decisions about where...