WASHINGTON – Democrats have all but written off at least three Senate seats — in North Dakota, Indiana and Arkansas — and at least six House seats in Tennessee, Louisiana, New York and elsewhere as they embark on a final-weeks advertising push to minimize congressional election losses.

Emboldened by their prospects, Republicans recently threw $1.3 million into West Virginia in hopes of winning a Senate seat that was long thought out of reach. It was the GOP’s latest move to expand a playing field already heavily tilting its way.

In the one-month dash to Election Day, both parties are zeroing in on races they have the best chances of winning, recalibrating strategies and shifting advertising money by the day. The state of play could change repeatedly between now and Nov. 2.

Democrats are especially worried about House districts in the economically troubled Midwest, and their chances of picking up GOP-held Senate seats have dwindled.

In the final stretch, the Democratic Congressional Campaign Committee has reserved at least $52 million to run TV ads in more than 60 districts, nearly all held by their own party. The National Republican Campaign Committee has set aside $35 million in airtime in 55 races, and officials say more is on the way.

The disparity is misleading. Democrats consistently have had a cash advantage, but GOP-allied groups have weighed in and advertised in crucial contests for weeks.

Control of Congress and the outlook for President Obama’s agenda is at stake this election. Some five dozen or more House races are competitive, mostly for seats now held by Democrats. Republicans need to win 40 to take control.

Of the 37 Senate races, about a dozen are close. The magic number for the GOP is 10.

“The political environment is positive for us. I think our candidates are strong. And really it’s going to be a resource issue now on how we can maximize the use of limited resources,” said Sen. John Cornyn of Texas, chairman of the Senate GOP’s campaign efforts.

His Democratic counterpart, Sen. Robert Menendez of New Jersey, predicted his party will retain its majority despite the tough landscape marked by “a lot of ups and downs.” He added: “I don’t think the roller coaster is ready to level out anytime soon.”

While technology has changed politics, television advertising still is a powerful and necessary political tool to reach voters, and it’s the best indicator of where party leaders think they have a shot at winning.

Both sides have reserved millions of dollars of airtime and will decide whether to cancel those orders or send the check. Any movement from one side will force the other to change strategy.

Democrats already appear resigned to losing at least six House seats. They include four districts left open by retirements and where no advertising is planned: Tennessee’s 6th, Louisiana’s 3rd, New York’s 29th and Kansas’ 3rd. Airtime has been set aside for two other open seats — Arkansas’ 2nd and Indiana’s 8th — but Democrats expect party leaders to cancel those plans if polls continue to show their candidates trailing badly.

Republicans, who are defending six competitive seats, expect to lose at least two: the open, at-large Delaware seat and Rep. Joseph Cao’s 2nd district seat in Louisiana.