The coming November rematch between Republican Sen. Ron Johnson and Democratic challenger Russ Feingold is narrowing, according to the latest Marquette Law School Poll.

The poll found Feingold leading Johnson 48-45 percent among likely voters. It was the first time the poll reported results based on those who say they are certain to vote in November.

Last month the poll found Feingold leading 49-37 among registered voters. In the poll released Wednesday, that lead shrank to 47-42 among registered voters.

Both campaigns said the poll results signify that the race will be close through election day. Feingold’s campaign worked that message into a fundraising appeal within an hour of the poll being released.

Poll director Charles Franklin said the tightening of the race could be a result of voters hewing more closely to their party as the presidential primary approaches.

“As always we have to wait until next time to see if it continues to tighten,” Franklin said.

Among registered voters, roughly a third have a favorable view of Johnson, another third have an unfavorable view and 36 percent say they haven’t heard enough to form an opinion, which is almost unchanged from February.

For Feingold it was 41 percent with a favorable view, 35 percent with an unfavorable view and 25 percent with no opinion, which was also very similar to last month.