One Republican group has reserved $6 million in television advertising time for the fall election season to help more than a dozen House GOP candidates, and about half the money will come from the nonprofit side of the organization that is not required to disclose its donors.

The Congressional Leadership Fund and its nonprofit affiliate, the American Action Network, reserved the ad time in seven key media markets — which will also be likely battlegrounds for the presidential race — as a down payment for what is expected to be a much larger fall campaign to promote House Republicans.

The move comes as congressional Democrats step up their criticism of nonprofit groups that shield their donors and that are playing an increasingly prominent role in House and Senate races.

On Monday evening, Senate Republicans blocked consideration of a Democratic bill that would require those nonprofits to disclose the donors of every contribution of at least $10,000 that is used for political purposes. The DISCLOSE Act, as the proposal is known, failed on a vote of 51 to 44, falling short of the 60 votes needed to proceed to a full debate.

Senate Majority Leader Harry M. Reid (D-Nev.) had tried, and failed, earlier to require nonprofits to reveal their donors after a 2010 Supreme Court ruling allowed corporations, unions and other special interests to spend money directly advocating individual candidates in elections.

That ruling prompted the creation of super PACs, political action committees that have collected six- and seven-figure checks from the wealthiest donors to push candidates in the presidential and congressional elections. For instance, one super PAC that supported former House speaker Newt Gingrich’s failed bid for the GOP presidential nomination took in $21 million from the Adelson family of Las Vegas.

All donations to super PACs must be disclosed. But most of these PACs also have an arm that is formed under the 501c(4) section of the tax code, which allows for a slightly less aggressive political posture and does not require donations to be revealed.

Democrats, who are courting super PAC donations themselves, have been demanding that all the contributions be disclosed. “We have proven it’s possible to remove the veil of secrecy of outside money and make the process more transparent. We’ve done it before; we need to do it again,” Reid said in a floor speech Monday.

Republicans and their corporate supporters reject this argument. In a recent letter urging opposition, the U.S. Chamber of Commerce said that disclosing all donations to such groups could open up businesses to “retaliation against unpopular or unfavorable political views, which also infringes constitutional rights.”

In its own undisclosed political activity, the Chamber has spent more than $3 million this year opposing Democratic Sen. Sherrod Brown’s reelection bid in Ohio, according to Democrats who have monitored TV ads there.

In the 2010 campaign, the American Action Network spent a reported $30 million on ads, with almost all of the money coming from undisclosed donors.

This year, the group — overseen by former senator Norm Coleman (R-Minn.) and former representatives Tom Reynolds (R-N.Y.) and Vin Weber (R-Minn.), and run by former House GOP aides — added the super PAC arm, the Congressional Leadership Fund. It has become the main outside group associated with the leadership team of House Speaker John A. Boehner (R-Ohio), who has raised funds for the super PAC.

Already active in many districts, the group has mapped out 14 critical House races in media markets — including Orlando, Las Vegas, Des Moines and Cleveland — that will also be key presidential battlegrounds. Several of the seats are of particular importance to Boehner, including the one in Des Moines, where his friend Rep. Tom Latham is in a tough reelection battle, and where the group reserved $330,000 in ad time. Another Iowa incumbent in a tough race, Rep. Steve King, could also stand to benefit from that air time.

With $1 million slated for Cleveland, two GOP incumbents in Boehner’s delegation will have outside resources defending their conservative records. With about $3 million, the American Action Network is targeting a pair of new districts, one outside Orlando and one outside Las Vegas; two New Hampshire Republicans stand to benefit from $650,000 of advertising reserved there; and a pair of freshmen — one from outside the Twin Cities and one just across the Wisconsin border — would benefit from nearly $900,000 in advertising for the Minneapolis-St. Paul market.

Of the 14 seats targeted, 10 are held by members of the huge GOP freshman class, including two in the expensive Philadelphia media market: Reps. Michael G. Fitzpatrick (Pa.) and Jon Runyan (N.J.). That market received the largest allocation of funds, $1.6 million, from the group.