RALEIGH, N.C. — The group dedicated to winning a Republican U.S. Senate majority in November says it’s more than doubling its investment supporting North Carolina GOP nominee Thom Tillis’ campaign to unseat Democrat Kay Hagan.

In what’s already considered the most expensive race in the nation, the National Republican Senatorial Committee said Monday it’s authorized spending another $6 million on ads in the final three weeks of the election. A spokesman for the NRSC’s independent expenditure arm didn’t disclose when TV ads would begin but said they would air statewide.

Hagan, Tillis and outside groups have spent $59 million altogether so far in the Senate race.

Both major-party candidates and their allies have turned up the volume on airwave campaigning. Hagan has unveiled three TV and radio ads since late last week.