Democrats eyeing a rare opportunity to pick up a U.S. Senate in a traditionally red state are buying television time in Indiana for the second consecutive week as they fight to maintain control of the chamber.

The Democratic Senatorial Campaign Committee will spend $525,000 in Indiana over the next week, beginning Tuesday. The spots will air on broadcast and cable stations, said a Democrat tracking ad buys who confirmed the numbers on condition of anonymity because the ads had not started running.

The group also is spending $630,000 this week in a close race in Nevada, the Democrat said.

Senate Democrats have seen their chances of maintaining their slim edge in the Senate improve slightly in the last few weeks. But outside groups on both sides of the aisle are greatly increasing spending in key races as the battle for the Senate enters its final weeks.

In Indiana, Democrats see a rare chance for a pickup in the deep-red state following Republican U.S. Sen. Richard Lugar's primary loss to state treasurer Richard Mourdock.

The Republican-aligned Crossroads GPS is spending $1 million in one week on Indiana airwaves with ads targeting Democratic Senate candidate Joe Donnelly. The latest Crossroads buy is far and away the largest of the Indiana race so far, but could easily be surpassed in the coming weeks.

Other conservative groups have spent $700,000 and $800,000 each in Indiana, while super PACs aligned with Senate Majority Leader Harry Reid have spent heavily on Democratic Senate candidate Joe Donnelly's behalf.

In Nevada, Democratic U.S. Rep. Shelley Berkeley is running against Republican U.S. Sen. Dean Heller for the seat vacated by former U.S. Sen. John Ensign, following news of his affair with a top staffer.