That's from to Silver's latest analysis of published Monday night. Silver noted that while there haven't been many shifts in key states for control of the Senate, the biggest shifts have happened in Colorado and North Carolina, where Sens. Mark Udall (D-CO) and Kay Hagan (D-NC) are facing challenges from Rep. Cory Gardner (R-CO) and North Carolina House Speaker Thom Tillis (R-NC).
Silver goes on to note that most of the gains for Democrats "have come from purple states."
"What's perplexing is that this has happened right as Democrats' position on the congressional ballot —probably the best indicator of the national mood— has deteriorated," Silver wrote.
Silver goes on to note that the change might be because Democratic super PACs have spent more money this cycle int hose states than the conservative counterparts that support Republicans.
"Whatever the reason, the GOP's path to a Senate majority is less robust than before," Silver concluded. "They still look pretty good int he "path of least resistance" states. But while West Virginia, Montana and South Dakota are extremely likely pickups, Alaska, Arkansas and Louisiana are not sure things. Meanwhile Republicans have fewer top-tier backup options, as states like North Carolina and Colorado have trended away from them. Republicans may need to decide whether to consolidate their resources."