STEELVILLE, Mo. — Missouri is on a political knife-edge. For weeks, Senator Barack Obama has been inching up in the polls here, and some analysts now believe he is slightly ahead.

Mr. Obama’s campaign had refused to concede this bellwether state despite remaining behind in the polls for months, manning dozens of offices across rural areas and sending some 2,000 volunteers door-to-door seeking support for him.

At this point four years ago, Senator John Kerry, the Democratic presidential nominee, had all but conceded the state, but the Obama campaign’s effort has been unrelenting. About 250,000 voters have been added to the Missouri rolls since January, many — perhaps most — in the urban areas of Kansas City and St. Louis, where Mr. Obama is expected to win by overwhelming margins.

“I’ve never seen this kind of infrastructure being laid down, registering new voters,” said Clint Zweifel, a Democratic state representative who is running for state treasurer. “It’s pretty overwhelming, the operation.”