It's growing increasingly difficult to believe in a future with Bernie Sanders as Democratic Party's presidential nominee, as the gap continues to widen between himself and front-runner Hillary Clinton.

Clinton continues to pick up victories – including wins in four states on Tuesday – while Sanders remains solidly in second place on the campaign trail.

But while Sanders is "feeling the Bern" of falling behind in the race, there is still time for him to catch up, according to Democracy Institute director Patrick Basham. However, he'll need to change up tactics if he is to do so, Basham says.

"It's hard to see how he gets beyond her without a change in the dynamic," Basham told CTV's Canada AM on Wednesday. He said Sanders must start going after Clinton the same way Donald Trump – or whoever else wins the Republican nomination – would be attacking her in the fall.

"He has to become more of an attack dog," Basham said. He also pointed out that Clinton could still be indicted for the email scandal currently working its way through the U.S. justice system, meaning there is a possibility Sanders could "take the reins" if that comes to pass.

"As long as that grey cloud is there, it makes sense for him to hang in there," Basham said.

Basham says Sanders will need to pull off a few upset wins in the American Midwest and Northeast, in states such as New York and Pennsylvania, if he is to have a chance of beating Clinton. He added that Sanders could hit the "motherlode of delegates" if he manages to take California, which currently skews toward Clinton.

"If he can pull an upset in one or more of these big states, then he remains viable," Basham said.

Basham also suggested Sanders might be holding back with his attacks on Clinton, in hopes of running as her vice-presidential nominee in the future. "If he goes after her really tough, and he fails, then he's probably blown any chance of being on the ticket with her in the fall," Basham said.

Sanders will also need to tap into some of the African-American electorate, which usually favours Clinton, in order to catch up.

"As long as she can hold onto that demographic, as she has so far, it's almost impossible for him to beat her," he said.