With four months to go until this year’s midterm elections, perhaps the only thing clear about the fight for the Senate is that it will pose challenges to public polling.

There’s always the possibility that the polls could miss the outcome in a close contest. Polls have missed the result in three close Senate races in the last two cycles. But this year is particularly challenging. The rapid growth of partisan polls has contaminated the polling averages in states where surveying public opinion is already difficult. Many of these partisan polls employ dubious weighting and sampling practices. The combination will make it even harder for polls to nail the result.

So far this year, 65 percent of polls in Senate battlegrounds have been sponsored or conducted by partisan organizations, and an additional 10 percent were conducted by Rasmussen, an ostensibly nonpartisan firm that leans conservative and has a poor record.

The situation is even worse in the five states with vulnerable incumbent Democrats, including Louisiana and North Carolina, which are likeliest to determine control of the Senate. There, 75 percent of polls are from partisan organizations, and Rasmussen conducted about half of the remaining 25 percent of polls. There have been only seven nonpartisan, non-Rasmussen polls in these five states in 2014.