The Democratic Senatorial Campaign Committee has reserved $5.1 million of fall airtime in Ohio, a sign they believe Sen. Sherrod BrownSherrod Campbell BrownLawmaker interest in NAFTA intensifies amid Trump moves Dem senator shares photo praising LeBron James after Laura Ingraham attacks Trump gets recommendation for steep curbs on imported steel, risking trade war MORE (D-Ohio) faces a serious threat from Ohio state Treasurer Josh Mandel (R).

The total, as relayed by a Republican operative tracking the buy, is so far only on broadcast television, meaning the buy's size is likely to increase once cable purchases are added.

Brown holds a single-digit lead over Mandel in most recent polls, and while the race is not yet considered a top GOP target, there have been increasing signs that both campaigns expect a tough fight in the state. Mandel's campaign has been a bit shaky out of the box, but heavy spending from GOP-aligned outside groups has buoyed his numbers and weakened Brown.

The buy could be more of a precaution than anything else, however — the committee can always cancel it and use that money elsewhere, and because Ohio is a top presidential swing state ad rates are likely to go through the roof as the election gets nearer, with a strong possibility that all ads in the state might be bought up before the election is over.

The ad reservation is for the Cincinnati, Cleveland, Columbus, Dayton, Toledo and Youngstown media markets.