Which party benefits from Johnson’s retirement?

This afternoon at 3 p.m., Sen. Tim Johnson is expected to announce his retirement.

Which party will benefit more from that announcement, compared to an alternative world in which Johnson sought reelection to a fourth Senate term?

Some say Republicans do. Jennifer Duffy with the Cook Political Report said even if the Democrats get a strong replacement candidate, it’s always harder to defend an open seat than to reelect an incumbent. Greg Strimple, a Republican pollster, told the Associated Press that Johnson’s retirement means “South Dakota moves into the top slot as the most likely Republican pickup.”

The recent Public Policy Polling poll would tell another story. Johnson’s approval rating is below 50 percent, even with the number of voters who disapprove of his job performance. Ex-Rep. Stephanie Herseth Sandlin has a much higher approval rating, while Johnson’s son Brendan, though largely unknown, is at least not broadly disliked. Couple this with the qualitative analysis that a younger, healthier Democrat might be able to wage a more vigorous campaign than Johnson, still struggling with the aftereffects of his 2006 brain hemorrhage.

What do you think? Is Johnson’s retirement a blow to Democratic hopes of holding this seat, or does it give them a better chance than they’d have with him on the ticket?