Sen. Bob Menendez, D-N.J., who faced ethics questions this year, is now at risk of losing his Senate seat.

The race that was seen as "lean Democratic" is now a toss-up race, according to election analysts at the Cook Political Report.

Menendez received a warning this year by the Senate Ethics Committee about receiving gifts from a wealthy campaign donor and then trying to advance that donor's interests in the Senate. And those ethics issues could be enough to unseat Menendez.

"The contest isn’t about anything else but Democratic U.S. Sen. Bob Menendez and his ethics problems," wrote Jennifer Duffy of the Cook Political Report Friday morning.

Duffy said the Republican in the race, pharmaceutical CEO Bob Hugin, has outspent Menendez more than 2-1 in the race. But Duffy said the bigger problem for Menendez is voters who might have had enough of him.

"The biggest threat to Menendez’s re-election is not so much Hugin than it is the voter who goes to the polls and decides to send Menendez a message, much the way many did in the primary when 38 percent voted for his unknown primary opponent," she wrote. "There is certainly a thumb on the scale for Menendez, who is said to have a lead of between four and six points, in this very blue state, but the race is close enough to warrant a move to Toss Up."

Duffy said shifting sentiment in New Jersey will make it even harder for Democrats to take control of the Senate. She said Democrats would have to hold all five Democratic races seen as toss-ups, and win three of the four GOP seats that are seen as toss-ups.

"That's just not a realistic scenario," she wrote.