National Democrats are laying down millions to defend Sen. Mark Udall (D-Colo.) (Susan Walsh/AP)

The Democratic Senatorial Campaign Committee began purchasing television advertising time in three key battleground states Thursday, setting aside more than $9 million for advertising set to start airing in August, while rival Republicans begin their own fall television ad buys.

The DSCC will spend $4.7 million in Colorado, where Sen. Mark Udall (D) finds himself in an unexpectedly difficult fight with Rep. Cory Gardner (R) for advertisements set to begin Sept. 16. The bulk of those ads will be spent in the Denver media market, with a smaller amount landing in the swing Colorado Springs-Pueblo market.

The National Republican Senatorial Committee began making their own Colorado ad buys Thursday, dropping $2.1 million on Denver advertisements set to begin running on Sept. 22.

Sen. Mark Pryor (D-Ark.) will receive about $3.6 million in late aid from national Democrats beginning Aug. 5. Most of the DSCC money is going into the Little Rock market, which covers most voters in the state, with smaller amounts designated for Jonesboro and the Fort Smith market, in the northwest corner of the state.

The NRSC had previously reserved about $1.75 million in late advertising in Arkansas, where Pryor faces freshman Rep. Tom Cotton (R).

The DSCC is also reserving $1 million in airtime in Manchester, N.H., were Sen. Jeanne Shaheen (D) faces former Sen. Scott Brown (R).

Senatorial committees begin buying or reserving last-minute airtime in key states months ahead of time to lock in low rates and guarantee certain targeted commercial spots. Both parties have already reserved millions of dollars in advertisements in Alaska, where Sen. Mark Begich (D) faces a difficult reelection challenge.

The two sides are likely to reserve millions more in other battleground states. Neither party committee has made initial forays into Louisiana, North Carolina, Iowa, Michigan, Kentucky, Montana or Georgia, the other top-tier Senate races up for grabs this cycle.