PARDEEVILLE, Wis. — Margo Miller thanked her hosts, covered herself in a blue plastic poncho and headed back into the driving sleet Saturday morning, with more doors to knock on in this rural subdivision but with another new volunteer’s name on her clipboard. Miller and about 50 other Democratic activists who braved the spring storm returned with 160 petition signatures for a special state Assembly campaign, a local sleeper election that Democrats hope will be anything but. Since Republican Donald Trump’s surprise win in Wisconsin helped put him in the White House, Miller and other Democrats have channeled their anger and soul-searching into races closer to home, racking up unexpected victories that have alarmed Republicans across the country. The epicenter of the Republican resurgence of eight years ago, Wisconsin is now the proving ground for a Democratic revival.