Elaine Marshall, North Carolina’s secretary of state, announced Wednesday that she has taken the first official step to launching a bid against Republican Sen. Richard M. Burr — giving the Democrats the top-tier challenger they have sought for one of their targeted 2010 Senate takeover bids.

Marshall filed paperwork with state officials establishing her intention to run. And Marshall campaign adviser Thomas Mills said the response in the 24 hours since has been “bigger than we thought.”

“We just about couldn’t handle what came in yesterday,” Mills said of the donations, phone calls and e-mails pouring in to Marshall’s nascent campaign. Democrats, he said, “want to have somebody to get behind against Burr.”