It has been a few days since Battleground270's Senate forecast has been updated. In that time, Democrats have received very little good news. In fact, in every race that has been polled since it was updated, the Democrats' position has deteriorated, if only slightly. However, all of those slight changes add up to a large increase in the GOP's chances of winning.
Since last Wednesday, the Republicans' chance of taking the upper chamber have risen from 56% to 64%.
What can be attributed to the large shift? Mostly, it is the races in the two "swingiest" states, Iowa and Colorado. In short, Democrats pretty much need to win these two states if they want to continue having a majority in the Senate. The only other options would run through states like Alaska and Arkansas, or perhaps even Georgia; all these states are rated as "likely GOP" at this point.
Over the weekend, Republicans got a boost with polls showing their candidates in Iowa and Colorado are leading their Democratic counterparts. The largest shift has come in Iowa, in which Battleground270 had the Democrat, Bruce Braley, with a 52% chance of winning. Now, however, he only has a 42% chance of winning.
Note: at this point in the cycle, with five weeks until the election, the model still give a relatively large amount of weight to a state's "fundamentals"; essentially, if polls show a tied race in a deep red state like Louisiana, the model will adjust that to assume a slight Republican advantage, and vise versa. As we get closer to the election, those built in factors will be diminished, and the polls will be given more weight.