LINCOLN, Neb. — For nearly a decade, opposition to former President Barack Obama’s health care law has been a winning message for Nebraska Republicans. It’s helped them win every statewide office, control the Legislature and hold all the state’s congressional seats. So it was something of a surprise for Bob Tatum when he set out to ask his fellow Nebraskans if they would back a ballot initiative to expand Medicaid, one of the pillars of Obama’s health overhaul. “There seems to be a lot more support than I anticipated,” said Tatum, who lives in a remote town near the Colorado border. It took him little more than a week to gather more than 100 petition signatures in Perkins County, where roughly 70 percent of the 1,963 registered voters are Republicans.