Despite her stratospheric poll numbers, the Democratic front-runner is likely to face a primary fight. (Frank Franklin II/AP)

With every passing week, one thing becomes ever clearer: Hillary Rodham Clinton is going to be the Democratic presidential nominee in 2016.

This past week’s evidence came in the form of two polls — conducted by NBC and Marist College — of Democratic voters in Iowa and New Hampshire. In Iowa, Clinton led Vice President Biden 70 percent to 20 percent. In New Hampshire, Clinton led Biden by 74 percent to 18 percent. (That’s not to pick on Biden; he was the strongest of Clinton’s possible challengers.)

Despite those stratospheric numbers, it’s a near-certainty that Clinton will face some sort — or sorts — of primary opposition. Which begs the question: Why?

To answer that, it’s important to remember that not everyone runs for president to win. Some run to promote a cause or a set of beliefs. Others run because timing dictates that they have to. Still others run in hopes of improving their chances of winding up on the ticket alongside Clinton or with a prominent spot in her administration.

When it comes to 2016, the largest group of potential challengers to Clinton comes from the “cause” category. Socialist Sen. Bernard Sanders (I-Vt.) seems intent on running, largely to push his belief in the need for serious campaign finance reform. Maryland Gov. Martin O’Malley is term-limited, with his second term ending in January, and undoubtedly thinks a credible run for president might bolster his chances of a spot in a Clinton administration. Former Montana governor Brian Schweitzer just seems to want to be back in the political game.

Below are my rankings of the 2016 field. Remember that if Clinton runs, she wins.

Tier 1 (The Clinton wing.)

Clinton: Still think she hasn’t made up her mind about running? Check out what Clinton told Charlie Rose in an interview last week: “We have to make a campaign about what we would do. You have to run a very specific campaign that talks about the changes you want to make in order to tackle growth, which is the handmaiden of inequality.”

Tier 2 (If she doesn’t run, these are the front-runners.)

Biden: The vice president badly wants to run. Just look at his travel schedule, which last week included a keynote address at Netroots Nation, an annual gathering of liberal online activists. And his allies insist that his decision on the race has nothing to do with what Clinton decides. But Biden didn’t get this far in politics by being dumb; a race against Clinton is damn close to unwinnable for him — and he knows it.

O’Malley: The Maryland governor is getting some nice press in early primary states. And he is working those states like no one else in the Democratic field. Because O’Malley can’t really afford to wait four (or eight) years to run, I expect him to be in the race no matter what Clinton does.

Elizabeth Warren: The senator from Massachusetts is the only person who could credibly mount a challenge to Clinton. But she’s not going to do it. Although Warren is on the record saying she will serve out her six-year Senate term, which expires in 2018, I am hard-pressed to see how she would forgo a run if Clinton took a pass.

Tier 3 (Maybe running. But not winning.)

Howard Dean: Dean has the presidential bug. In 2004, he looked as if he was going to be the Democratic presidential nominee — until people started voting. In 2013, Dean predicted that Clinton would have a primary opponent, and he may see himself as that person.

Sanders: Of everyone not named Clinton (or O’Malley) on this list, the senator from Vermont is doing the most to get ready for a presidential bid. No one — including Sanders — thinks he will win, but his liberal positions could make things uncomfortable for Clinton.

Schweitzer: The former Montana governor proved in a recent interview with National Journal’s Marin Cogan that he isn’t yet ready for prime time, making impolitic comments about Sen. Dianne Feinstein (D-Calif.) and Rep. Eric Cantor (R-Va.). Schweitzer apologized, but the damage was done. He effectively doused any momentum he had built for a presidential bid.

Tier 4 (The four- or eight-more-years crowd.)

Andrew Cuomo: The New York governor has a presidential bid in him, but it’s not going to be in 2016. If anything, he has moved farther away from a bid rather than closer to one. In 2020, Cuomo will be 62 — right in the sweet spot when it comes to presidential bids. In 2024, he will be 68, the age Clinton will be in 2016.

Kirsten Gillibrand: Like Cuomo, Gillibrand is an ambitious New Yorker who almost certainly will run for president at some point. At 47, she has plenty of time to wait and, as she has done over the past few years, use her perch in the Senate to build her liberal résumé for an eventual national bid.

Deval Patrick: Patrick raised some eyebrows a few months back when he had this to say about Clinton’s coronation as the Democratic nominee: “She’s an enormously capable candidate and leader. But I do worry about the inevitability thing, because I think it’s off-putting to the average . . . voter.” It seems very unlikely that the Massachusetts governor will take the plunge against Clinton, but his résumé in the Bay State could make for an intriguing profile in four or even eight years.