Democrats in key Senate races have been breaking records with their fundraising — signaling they'll have substantial resources available to them in the final weeks of the election season. And yet, last month, Republicans outspent them on air, largely thanks to outside groups.  The shift in spending has come up on the campaign trail. It hasn't escaped the notice of the Democrats that they're getting hammered by GOP ads, and they may be having an effect: polls have tightened in recent weeks in multiple battleground races. Ad spending in the general election for the Senate has now topped more than $1 billion, and more than half of that has gone into four races: Georgia, Nevada, Pennsylvania and Wisconsin. When it comes to the Senate races, Democrats have been able to exceed or match Republicans on the airwaves in terms of the reach of their ads in most weeks since the general election began, according to a source tracking media buys. That's in part because candidate campaigns receive bett