"It's going to be extremely challenging," Manchin said on CNN's "State of the Union."
He said that the success or failure of the health care law will be a major factor in the 2014 elections.
"It's weighing heavy on that," he said. "They think that that's a great advantage that they have," he added about Republican challanegers.
Manchin, a moderate Democrat, expressed skepticism about Obamacare's success so far, which has lead him to work on a delay of the individual mandate.
"Yes, we're moving the ball forward," he said, but added that he's concerned about how well the law has been received. "I'm worried about having a product that's a marketable buy."