David Corn has a pretty smart take on how Hillary Clinton needs a real opponent in the Democratic nomination contest, and points to former Maryland Governor Martin O'Malley as the most likely person to fill the role:
Why Hillary Clinton Needs Martin O'Malley to Run for President
...Clinton, who finally jumped into the 2016 race with a tweet and video splash on Sunday, could benefit if she is challenged in her party's primaries by O'Malley or someone else.
There's about nine months to come before the first voting occurs in the Iowa caucuses—and 19 months until the general election. That's a long time. Clinton, who is hardly a fresh face, will find it tough not to appear stale to some voters during that stretch. She is already at a super-saturation level of media coverage. There are endless tweets, blog posts, and articles about every aspect of her campaign. All her moves—her logo!—receive inordinate press attention. Though she and her aides insist this race is not about her—it's about everyday Americans and how to improve their lot—the campaign is likely to be much ado about Clinton: how she campaigns, what she says, what's her vision, where she goes, how she's performing, what's her strategy, what's up with her husband, her connection with voters, her trustworthiness, her likability, and so on. Her every utterance and move will be dissected everywhere—again and again. (And the various dissections will be dissected.) On the Republican side, all the 2016 wannabes will be directing attention at her, as they each angle to be seen as the candidate best able to obliterate Clinton. Sure, the GOPers will eventually form a circular firing squad—they won't be able to resist the urge to attack one another—but they will direct many shots at Clinton. The around-the-clock Hillary Bashathon will never end.
It would be tough for any candidate to withstand this degree of hyper-scrutiny for such an extended period...
Clinton needs a foil in the Democratic primaries—someone she can joust with, someone who will expand the narrative, and someone she can beat. Waltzing through one election after another will not boost her commander-in-chief credentials. A fight or, at least, a tussle—even a lopsided one—will give her campaign more of a story to tell, and, presuming she wins the primaries, will position her as, well, a winner, not a candidate who is skating toward the general election on the easy ice of entitlement and inevitability. Barack Obama's ability to dispatch Clinton in 2008 demonstrated his moxie and his mettle. His glow intensified with each victory. Everyone likes a winner, right? And these battles were great training for the match-up to come against Republican John McCain. Clinton will not face as formidable a primary foe as Obama did. But a face-off against any opponent of consequence is better than a breezy promenade toward the main event.
O'Malley, who's considering a presidential bid, would make a good sparring partner...
Corn makes some excellent points, and anybody watching the 2016 Democratic nomination process should read the whole piece here: Why Hillary Clinton Needs Martin O'Malley to Run for President