"We take full responsibility for this situation," the statement said.
Databases containing data on 198 million potential US voters were exposed online without the need for a password, allowing anyone to download the information.
The unsecured server was first uncovered last week by security analyst Chris Vickery and the firm confirmed to the US news blog Gizmodo that it was authentic.
According to media reports, one file called "Post Elect 2016" contained information on voters’ likely views about topics such as US President Donald Trump’s "America First" policy and whether they voted for former President Barack Obama.
The database also contains files that may be from another data firm, TargetPoint, suggesting the two companies shared US voter information.
The RNC paid Deep Root more than $983,000 between January 2015 and November 2016, and paid another $4.2 million to TargetPoint.