With her current problems anyone would be a good rival.
Check it out:

Former Maryland Gov. Martin O’Malley is emerging as a potential challenger to Hillary Clinton for the party’s 2016 presidential nomination but appears unwilling, at least for now, to mount a head-on challenge to the front-running Clinton.

O’Malley on Friday night at a Democratic fundraiser in key voting state New Hampshire declined to discuss two Clinton controversies — donations to the Clinton Foundation and her use of a private email accounts — much less use them to his political advantage.