Sen. Bob Casey (D., Penn.) could be in a vulnerable position for his 2018 reelection campaign after a new poll released on Thursday signaled that half of Pennsylvania voters believe he has done only a "fair" or "poor" job as senator.

Of the voters surveyed, 33 percent believe Casey is doing a "fair" job and 17 percent believe he is doing a "poor" job, compared to 28 percent who believe he is doing a "good" job and 10 percent who believe he is doing an "excellent" job, according to the Franklin & Marshall College poll.

The poll results are a sign that Casey could be facing a tough reelection fight next year.

Some of his vulnerable Democratic colleagues have been treading softly in the current partisan environment and attempting to reach across the aisle, but Casey has been part of the so-called "resistance" movement against President Trump. Casey called Trump's firing of FBI Director James Comey "Nixonian" on Tuesday, and he told CNN last week that "nothing gets me to a yes" on the Republican health care plan.

This is Nixonian. Deputy AG Rod Rosenstein must immediately appoint a special prosecutor to continue the Trump/Russia investigation. — Senator Bob Casey (@SenBobCasey) May 9, 2017

.@SenBobCasey says "nothing gets me to a yes" on GOP health plan, "I'll fight to the end of the earth" https://t.co/tn64HoOOlq — CNN Newsroom (@CNNnewsroom) May 4, 2017

Casey's "poor" job rating has increased by three percent since February, which could suggest that voters do not support his approach to working with the Trump administration. Trump received 48.2 percent of the vote in Pennsylvania last November against Hillary Clinton.