Nate Silver is handicapping the 2010 U.S. Senate races at Fivethirtyeight.com and had this to say about Iowa’s seat, held by five-term incumbent Chuck Grassley:

Grassley will be 77 in 2010 and could retire, in which case the race probably leans Democrat. Absent a retirement, a kamikaze mission by someone like Tom Vilsack against the popular incumbent is unlikely to succeed.

Over at Iowa Independent, Chase Martyn begs to differ:

Grassley has not had a truly difficult race in some time.  […]

In 2004, Art Small […] received no institutional support from the Democratic party, which essentially conceded the race before it began.

In 2010, the picture is very different.  While Grassley’s approval rating remains high, almost everything else has changed.

Democrats have begun to truly dominate Iowa’s political scene. […]

What happens if former Gov. Tom Vilsack jumps into the race for Senate?

Fending off Vilsack’s challenge, Grassley could face deficits in both fundraising and name identification for the first time in decades. […]

Far from a ‘kamikaze mission,’ as Silver calls it, the emerging conventional wisdom around here is that Vilsack would have a real chance against Grassley in 2010.

Perhaps “kamikaze mission” is too strong a phrase, but we need to acknowledge that Tom Vilsack or any other Democrat would be a serious underdog against Grassley. Yes, Iowa now has far more registered Democrats than Republicans (about 106,000 more, last I heard), but Grassley has always benefited from a strong crossover vote.

Grassley will face substantial pressure not to retire in 2010, in part because several other Republican-held Senate seats are likely to be vulnerable. Furthermore, Iowa Republicans hoping to unseat Governor Chet Culver would love to be able to focus their spending on that campaign, rather than divide their resources between the gubernatorial race and defending an open Senate seat.

As I see it, four factors could push Grassley toward retirement:

1. A health problem (God forbid).

2. An unpleasant 2009 in the Senate minority. Grassley loves his job and has gotten along well with Montana Senator Max Baucus, who chairs the Senate Finance Committee. But what if the enlarged and emboldened Democratic majority doesn’t need to cut as many deals with Grassley as Baucus has done in the past?

3. Deteriorating relations between Grassley and the social conservatives who dominate the Republican Party of Iowa. For background on this tension, click here or click here.

4. A top-tier Democratic challenger who can raise a lot of money and has free time to campaign.

And that brings me to the paradox in the title of this post. Clearly Grassley’s retirement would give Democrats the best chance (some might say only chance) to win this seat. However, Grassley is more likely to retire if Tom Vilsack or another major-league Democrat jumps in now, instead of waiting a year or longer to see whether the incumbent will decide to step down for some other reason.

Challenging Grassley means embarking on long and exhausting uphill battle. But putting Grassley on notice soon that Democrats will not give him a pass is one of the few things we could do to improve the odds that he will retire.

What do you think, Bleeding Heartland readers? Can Iowa Democrats recruit Vilsack or another top-tier challenger for this race?