According to the former New York Times statistician, Republican odds stand at 55 percent, a drop from 64 percent just two weeks ago.
"We've never quite settled on the semantics of when to call an election a 'tossup.' A sports bettor or poker player would grimace and probably take a 55-45 edge. But this Senate race is pretty darned close," Silver said on an article on his website.
Silver's detailed statistical model indicates that Democrats now have a stronger possibility of winning due to changes in the Senate races in Colorado and North Carolina which are currently giving the party an advantage when previously Republicans held the edge.
Silver categorizes the two states as "highly competitive purple states," among which are also Iowa, Michigan, and New Hampshire. All five seats are currently held by Democrats and, with the exception of New Hampshire, it has been in these areas where Democrats have gained ground.
He cited numerous recent polls that have shown a surge for North Carolina Sen. Kay Hagan and Colorado Sen. Mark Udall.
"What's perplexing is that [the Democrat surge in purple states] has happened right as Democrats' position on the generic congressional ballot — probably the best indicator of the nation mood — has deteriorated," he said.
He added that unlike the most recent figures, average historical data from the generic ballot tended to directly correlate to performance of candidates in state-by-state Senate polls.
Silver said the influence of money in those races could be one explanation, citing massive financial advantages for the Democrats in North Carolina and Colorado, along with higher outside spending by Democratic-leaning super PACs.
"Whatever the reason, the GOP's path to a Senate majority is less robust than before."