President Trump’s political operation is behind a mysterious new super PAC that on Tuesday placed a $25 million reservation for advertisements to help Republicans in half a dozen key midterm races. The new group, No Going Back PAC Inc., is linked to MAGA Inc., Mr. Trump’s powerful main super PAC, according to two people with knowledge of the matter who insisted on anonymity to disclose the private spending plans.

The reservations target six Senate races and one House battleground, with most of the money going to the Senate contests in New Hampshire and Alaska. The spending represents the biggest midterm expenditure yet tied to Mr. Trump’s $400 million super PAC, which his team has promised for months will spend significantly on this year’s races. It also comes on the eve of the president’s unusual two-day convention in Dallas.

No Going Back PAC Inc. was incorporated just a week ago, on Sept. 1, ensuring that it will not be required to file paperwork disclosing its funders or founders until mid-October. The group was incorporated by Charles Gantt, a Republican political operative who specializes in campaign finance and regularly handles paperwork for Trump-connected entities, including MAGA Inc. Throughout Tuesday, the group placed $25.8 million in advertising reservations, with some ads beginning to run on Wednesday.

The spending on Senate races includes $9.6 million in New Hampshire and $7.7 million in Alaska, according to Ad Impact, a media tracking firm. The group also dedicated smaller amounts to Senate races in Michigan, Ohio, North Carolina and Georgia, as well as a competitive House race in Pennsylvania where Representative Scott Perry, a Republican, is aiming to fend off a Democratic challenger.