In the days after Kamala Harris announced her presidential bid, the Democrats raised more than $100 million. Now, Donald Trump's team have filed a complaint with the Federal Election Commission, challenging whether she can take over Joe Biden's campaign funds.

Where does all this money come from? And what's it used for? This week we're joined by Sarah Bryner from OpenSecrets, an organisation dedicated to tracking the money in US politics to find out why presidential candidates are spending more than ever before on their campaigns, and what motivates people to donate to their cause.

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This episode was made by George Dabby with Rufus Gray and Claire Betzer. The technical producer was Mike Regaard. The series producer is Purvee Pattni. The senior news editor is Sam Bonham.