Early turnout looks similar to 2018, the "blue wave" year. The lab says enthusiasm appears to favor Democrats, who account for 44% of early votes compared with about 31% for Republicans.

Michael McDonald, a University of Florida political scientist, says the country could beat the 2018 turnout rate, the highest for a midterm since 1914. He also sees uneven patterns. Registered Democrats and unaffiliated voters are engaged right now, he said, while registered Republicans are not as much.

Early voting has climbed among young voters and minorities. McDonald believes this could be the most diverse midterm electorate in U.S. History, with high engagement among African Americans, Hispanics and Asian Americans, and less among whites.

Those numbers come with limits. Not all states require party registration, and some put little emphasis on early voting. McDonald expects Maine, Iowa, Kansas and Nebraska to cast maybe a third of their votes early, so they will look very different from vote-by-mail states. He says Republicans will make up some ground on Election Day, and nobody announces results until the polls close.