The rise of Donald Trump and the general "polarization" of politics have pushed six Senate seats toward the Democrats, according to Larry Sabato and his "Crystal Ball" forecast.

Sabato's updated forecast bodes particularly ill for Sens. Rob Portman and Pat Toomey, Republicans who are defending their turf in major purple states. Because there is a strong correlation between the success of a presidential candidate and Senate hopefuls of the same party, Sabato argues that the GOP could endanger itself if it nominates a general election candidate unpalatable to the general public.

"Considering the rise of Donald Trump, the polarization in U.S. politics, and a higher rate of straight-ticket voting, this could be bad news for the GOP," Sabato writes.

"Assuming the GOP nominee for the White House is either Trump or Ted Cruz, we think the Democrats will fare reasonably well down-ballot (more so with Trump than Cruz, though Cruz will also have a difficult time carrying many swing states)."

Portman and Toomey's seats are now rated as "toss-ups", a move from "leans Republican". Sens. Roy Blunt and Richard Burr are now just slight favorites. And even Sen. Chuck Grassley, an Iowa institution and chairman of the committee that oversees judicial nominations, is no longer a sure bet to win a seventh term, though his shift from "safe" to "likely" reelection was made "mostly out of an abundance of caution", Sabato writes. Grassley has won more than 60 percent of the vote in his five reelection campaigns.

The only seat currently held by Democrats that was reevaluated belongs to Coloradan Michael Bennet. That race moves from "leans Democratic" to "likely Democratic".

Republicans currently hold the Senate with 54 seats. However, of the 34 Senate races being contested in 2016, Republicans are playing defense in 24 of them.