WASHINGTON (AP) — High-dollar donors’ favorite political machine on Tuesday began booking another $20 million in television air time for an autumn advertising blitz aimed at tipping six Senate seats into Republican hands.

The Karl Rove-backed American Crossroads super PAC and its affiliated nonprofit Crossroads GPS disclosed they had requested blocs of ad time to start in September and to build on criticism against vulnerable incumbent Democratic senators. While Crossroads cannot directly coordinate with candidates or the GOP’s central committees, by making its advertising schedule public, allies can plan their media strategies.

Crossroads, which accepts unlimited donations and has some of the GOP’s top strategists as advisers, also plans to spend more cash on House campaigns.

Crossroads’ ad plan is just the latest example of high outside spending that now defines campaigns for both parties. The Senate Majority PAC has already aired more than $20 million in ads to help Democratic incumbents — many of them Crossroads’ targets. House Democrats’ and Republicans’ campaign committees, meanwhile, have booked almost $75 million in ad time for their fall campaign.

The biggest Crossroads ad booking so far is in Alaska, where American Crossroads plans to spend $5.5 million starting Sept. 9 to make Democratic Sen. Mark Begich’s re-election bid more difficult. Crossroads GPS also plans a $5.1 million ad campaign starting Sept. 30 in North Carolina to criticize Democratic Sen. Kay Hagan.

American Crossroads, which must disclose its donors, plans a $3.1 million ad campaign in the race to replace retiring Democratic Sen. Tom Harkin of Iowa and $1.7 million in Montana, where appointed Democratic Sen. John Walsh is running to stay in the seat.

Crossroads GPS, which does not disclose its donors, plans $2.1 million on negative ads in Louisiana about Sen. Mary Landrieu’s record and $2.5 million in Arkansas against Sen. Mark Pryor. Both are endangered Democrats.

The bookings, which can still be changed, are on top of the $9.4 million in ad time already booked in North Carolina, Colorado, Alaska and Arkansas through August.

The ad reservations suggest Crossroads has stepped up its fundraising. American Crossroads ended May with $4.6 million in the bank and has raised a cumulative $11 million since January 2013.

Crossroads GPS does not have to disclose its finances because under campaign finance rules it is considered nonpartisan and nonprofit.

Crossroads spent heavily on races in 2012 and came up short; 11 of the 13 Senate races where Crossroads spent money were won by Democrats, and GOP presidential nominee Mitt Romney failed to defeat President Barack Obama.

In all, American Crossroads spent more than $116 million between Jan. 1, 2011, and Dec. 31, 2012. During the peak of the 2012 campaign, American Crossroads spent $42 million between Oct. 18, 2012, and Nov. 26, 2012.

But Republican donors are looking at a Senate map that increasingly gives them optimism — and, thus, reason to open their wallets. Republicans need to pick up six seats to reclaim the majority for the first time since 2006’s elections.

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