I've encountered this claim in a number of sources, both respectable and not, but ultimately seems to stem from an article in Jerusalem Post attributing it to Gil Troy:
The reason for such intense scrutiny is the outsize contributions of Jewish donors to US political campaigns, with Jewish donors contributing a whopping 50% of funds received by the Democratic Party and 25% to the Republican Party, Troy says.
Gil Troy seems to be a indeed a professor of history in McGill University, but I failed to locate any paper that supports the data he provides, and on the face of it it seems a bit suspicious. Given how many donations go through PACs, unions, etc. it would be hard to have exact attribution for whole half of the money, let alone attribute it to a group that is just 2% of the electorate. So, is there any data (and description of methodology) to support this claim, or, alternatively, to refute it?