Democratic former Sen. Russ Feingold launched his second campaign ad in as many days on Thursday, putting his tally at five ads to Republican Sen. Ron Johnson's one.

Meanwhile, advertising placed by outside groups hoping to influence the race stacks up heavily in Johnson's favor. But none of that seems to have made much of a difference in the electorate's opinion so far.

A Marquette University Law School poll released Wednesday shows Feingold leading Johnson, 45 percent to 41 percent. His lead is one point smaller than it was in March.

Among likely voters, Feingold's lead is even larger, at 51 percent to Johnson's 42 percent.

Meanwhile, name recognition and favorability ratings for both candidates have remained relatively unchanged throughout the race.

It's early to read much into likely voters' inclinations, because voter enthusiasm is subject to fluctuation in the months leading up to an election. The Marquette Poll emphasizes registered voters rather than likely voters until later in the summer, poll director Charles Franklin said.