Former U.S. Sen. Scott Brown, now running to return to the Senate, hasn't officially made it through the Republican primary, but he has long been considered the front-runner to take on incumbent U.S. Sen. Jeanne Shaheen, D-N.H., in the general election in November. Brown has been the subject of numerous attack ads by outside groups, including a recent ad by the Senate Majority PAC, a Democratic-aligned group, that claimed he voted to give oil companies huge tax breaks and, in exchange, took more than $400,000 in campaign contributions.