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Wisconsin’s U.S. Senate race remains fluid and under most voters’ radars, according to the latest Marquette Law School Poll, though depressed Republican voter enthusiasm is giving the Democratic challenger a notable advantage.

Democratic challenger Russ Feingold leads Republican Sen. Ron Johnson 45-41 among registered voters, the narrowest margin since the question was first asked last year.

However, among likely voters, Feingold’s lead expands to nine points, 51-42.

Likely voters are those who say they are absolutely certain to vote in November.

Poll director Charles Franklin said that the divergence between registered and likely voters was noteworthy, particularly because only 78 percent of Republicans said they were absolutely certain they would vote in November, down from 87 percent in March and 90 percent in June 2012.

Among Democrats, 84 percent said they were certain to vote, up from 81 percent in March.

Franklin said those shifts illustrate the fluidity of the race at this point.

The poll found little change in voters’ views of Feingold and Johnson. Feingold is viewed favorably by 40 percent of registered voters and Johnson is viewed favorably by 33 percent.