Story highlights The video mentions that every single presidential nominee since 1976 has released his tax returns

Saturday's clip continues a line of attack Clinton has deployed against Trump this week

Washington (CNN) Hillary Clinton's campaign is keeping up its attacks on Donald Trump's refusal to release his tax returns.

The former secretary of state's campaign on Saturday released a web video asking what the presumptive GOP nominee could be hiding in the documents. The clip mentions that every single presidential nominee since 1976 has released his tax returns, before showing occasions where Trump has previously said he'd release them.

The video shows Trump saying he'd release his tax returns if President Barack Obama released his birth certificate and if he ran for office. Both conditions have been met -- but Trump has yet to release his returns.

The clip also shows Trump saying he is thinking about releasing his tax returns and another occasion when he said he can't because he is under audit.

Saturday's video continues a line of attack Clinton has deployed against Trump this week. Speaking in New Jersey on Wednesday, Clinton asked why Trump has not put out more information about how much he makes and pays in taxes.

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