After a week of holiday barbecue, hometown parades and constituent fence-mending, members of the U.S. Congress begin to drift back to Washington on Monday for what promises to be its most severe test of the year — finding common ground on a mammoth overhaul of the U.S. healthcare system.

The Senate is back in session on Monday afternoon and the House of Representatives returns on Tuesday to begin work on melding two different Senate bills and three House versions into legislation that can earn initial approval from each chamber before lawmakers adjourn for the month of August.

There are plenty of obstacles for the proposals, President Barack Obama’s top legislative priority this year, from trimming the potential $1 trillion cost to determining how to pay and whether to include a government-run public insurance option for approximately 46 million uninsured Americans.

Obama, meanwhile, arrived in Moscow on Monday as the United States and Russia try to break a stalemate on nuclear arms control talks. It is the opening of a weeklong Obama trip that also will include the summit of G8 wealthy industrialized nations in Italy and his first visit as president to Africa with a stop in Ghana.

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Photo credits: REUTERS/Hyungwon Kang (Fireworks over U.S. Capitol); REUTERS/Anatoly Maltsev/Pool (Obama and Russia’s President Dmitry Medvedev at Kremlin)