The billionaire investor, who scaled back his political giving after a then-unprecedented $27 million spending spree to try to defeat President George W. Bush in 2004, has quietly reemerged as a leading funder of Democratic politics — and as a leading boogeyman of conservatives.

Soros has donated or committed more than $25 million to boost Hillary Clinton and other Democratic candidates and causes, according to Federal Election Commission records and interviews with his associates and Democratic fundraising operatives. And some of his associates say they expect Soros, who amassed a fortune estimated at $24.9 billion through risky currency trades, to give even more as Election Day nears.

The 85-year-old Hungarian-born New Yorker had planned to attend his first-ever Democratic convention here to watch Clinton, with whom he has a 25-year relationship, accept the Democratic presidential nomination on Thursday. But an associate said he decided to cancel the trip this week because Soros, who recently returned to active trading, felt he needed to closely monitor the economic situation in Europe.