Story highlights The Hillary Clinton campaign will spend an addition $1.5 million in Iowa and $2.6 million in New Hampshire on ads

Recent polling has shown Vermont Sen. Bernie Sanders closing in -- and at times passing -- Clinton in those two states

(CNN) Hillary Clinton's presidential campaign plans to spend more than $4 million reserving ad time in Iowa and New Hampshire throughout September and October, aides said Thursday.

Facing stepped up questions about her ongoing email controversy and the prospect of Vice President Joe Biden getting in the race, the campaign will spend $1.5 million reserving ad time in Iowa, the aides said, and will spend an additional $2.6 million in New Hampshire.

The Iowa ad buy will focus on the Des Moines and Cedar Rapids media markets, while the New Hampshire one will be in the Boston, Manchester, New Hampshire and Burlington, Vermont, media markets.

Clinton became the first top-tier candidate to hit the airwaves when the campaign made a $2 million ad buy for five weeks starting in August. Thursday's ad buy was in addition to that and will focus on introducing Clinton to voters and conveying how her presidency would build on her life's work.

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