Sen. Ron Johnson is in serious jeopardy of losing his Senate seat to Democratic challenger Russ Feingold, a new poll shows.

According to a Wisconsin Survey poll released Wednesday night, Feingold leads Johnson by 10 points among registered voters, 51 percent to 41 percent.

While it is still early in the race, the numbers reveal just how hard the climb will be for Johnson, a freshman senator and chairman of the Senate’s Homeland Security committee. Johnson is facing re-election in a year that may prove difficult for Republican incumbents. Many Senate predictors are forecasting Republicans will have a tough time holding their majority especially if Donald Trump or Ted Cruz is the GOP’s presidential nominee.

Johnson is one of the most vulnerable senators up for re-election. The Wisconsin Survey found that 42 percent of registered voters surveyed approved of the job he was doing in Washington while 35 percent disapproved.

The poll, which was conducted for Wisconsin Public Radio by the Strategic Research Institute at St. Norbert College, surveyed 616 registered voters by phone between April 12-15. The margin of error is 4 percentage points.