Mitt Romney requested at least ten years of tax returns from several of his potential running mates during the vice presidential vetting process, three Republicans familiar with the requests confirmed to CNN.

One GOP source said that while Romney officials asked for a decade's worth of tax documents, they were "were willing to settle for less" as they scrutinized the financial backgrounds of several Republicans before Romney ultimately decided to tap Wisconsin Rep. Paul Ryan as his ticket mate.

The fresh details about the tax requests, first reported Thursday by The Daily Beast, threaten to complicate Romney's message as he continues to reject calls from President Obama's campaign team to release more than just two years of his own tax returns.

Democrats say Romney, who accumulated a substantial fortune during a nearly two-decade career in consulting and private equity, needs to show voters what tax rate he paid and provide more details on financial assets held in offshore bank accounts