WASHINGTON, May 7 (Reuters) - President Barack Obama's budget proposal for 2010 would spend $1 billion to endow a new federal fund meant to expand housing opportunities to low-income borrowers. The Housing Trust Fund was conceived by Rep. Barney Frank, chairman of the House Financial Services Committee, and was supposed to be funded by a tax on mortgage-finance agencies Fannie Mae (FRE.N: Quote, Profile, Research) and Freddie Mac. (FRE.N: Quote, Profile, Research)