Story highlights Bernie Sanders is still receiving protection from the Secret Service at a cost of about $40,000 a day

A Sanders spokesman declined to discuss why the senator has not relinquished his Secret Service protection

Washington (CNN) Bernie Sanders is back to his old day job, trading the booming applause of his campaign rallies to the far more tedious work of the Senate.

"I have a parliamentary inquiry!" Sanders shouted on the Senate floor one night last week, testing the patience of his colleagues, who were eager to leave the Capitol and start their holiday break.

But just off the Senate floor and across the Capitol, one vestige of his presidential campaign remains: his Secret Service detail. And taxpayers are footing the bill.

Protecting a presidential candidate costs about $40,000 a day, a federal official familiar with the Homeland Security budget told CNN. For Sanders, that's more than a half-million dollars since the last primary on June 14. The cost could grow by nearly $2 million if he stays in the race through the Democratic convention in Philadelphia later.

The federal official said it's difficult to tally exact costs, since some agents are working on other projects simultaneously, but the overall amount spent on Sanders is far higher when calculating the weeks of protection he received after the nomination was effectively out of his reach, as Hillary Clinton surpassed him in the delegate count.

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