The battle for the Democratic nomination is nearing crunch time with roughly one month left before the Iowa caucuses.

Some big-name candidates have dropped out - most notably Sen. Kamala Harris (D-Calif.) - while former New York City Mayor Michael Bloomberg has joined the field.

Who has the best chance of winning the nomination?

1. Former Vice President Joe Biden (July ranking: 2)

Biden has led national polls since he launched his campaign in April.

His advantage has eroded since then, but it is still significant. The former vice president is roughly 10 points ahead of his nearest challenger, Sen. Bernie Sanders (I-Vt.), in the polling averages of both RealClearPolitics (RCP) and data site FiveThirtyEight.

Biden's support has proved more resilient than many observers predicted, despite halting debate performances, attacks from rivals and smears from President Trump.

There are at least three pillars buttressing Biden's candidacy.

First and most important, he is way ahead of his rivals in terms of black support.

Second, polls from battleground states have helped him make the case that he is the best person to take on Trump.

And third, Biden's long career and centrist ideology appear to have a comforting appeal to voters who are tired of the never-ending dramas of the Trump era.

Biden has serious challenges too.

He is trailing in polls in both Iowa and New Hampshire. If he finishes off the pace in both states, what will happen to his argument about electability?

It's also plausible that part of Biden's appeal lies in name recognition - and that this advantage will be eroded as Democratic voters who are not political obsessives tune in to the race in coming weeks.

Biden is a relatively weak front-runner. But he is the front-runner nonetheless.

2. Sen. Bernie Sanders (I-Vt.) (July ranking: 4)

When Sanders had a heart attack in October, it could have spelled disaster for a candidacy that was already in danger of being eclipsed by Sen. Elizabeth Warren (D-Mass.).

But Sanders bounced back with vigor, delivering an impressive debate performance in Ohio, his first major appearance after his heart attack, while Warren has faltered.

Sanders's candidacy also got a positive jolt when progressive icon Rep. Alexandria Ocasio-Cortez (D-N.Y.) endorsed him. The 30-year-old congresswoman has appeared with the 78-year-old senator at several rallies, helping him draw huge crowds.

The passion of Sanders's supporters is also evident in his fundraising numbers.

When the last quarter ended, he had more cash on hand - $33.7 million - than any other candidate in the field. It is easy to imagine him holding on to the top spot in the money race when new financial filings come out soon, covering the last three months of 2019.

Sanders is narrowly leading in the polling averages in New Hampshire, the second state to vote. In Iowa, he is right on the heels of South Bend, Ind., Mayor Pete Buttigieg (D).

Sanders has his vulnerabilities too. Ill feeling lingers in some quarters of the party from his long 2016 campaign against eventual nominee Hillary Clinton. Centrists frequently voice fears that he could be too far to the left for the nation at large.

Even if Sanders won both Iowa and New Hampshire, the party establishment would almost certainly try to block his path to the nomination.

But there is no guarantee the centrists would succeed. Sanders has a real shot of becoming the nominee.

3. Sen. Elizabeth Warren (D-Mass.) (July ranking: 1)

Warren was the clear front-runner as recently as October, when she led polls in Iowa and New Hampshire, and challenged Biden for the national lead.

Her surge provoked scrutiny - and fierce attacks from her rivals. The result has been a dip from those halcyon days.

The Massachusetts senator's position on "Medicare for All" played a central part in her decline.

Some rivals, such as Buttigieg, argued that this was a more sweeping proposal than the electorate wanted. But Warren also failed to placate anyone by taking a half-step back, announcing she would not try to enact her plan before her third year in office.

There is some evidence that Warren also saw some of her support move to Buttigieg. Even though the South Bend mayor is more centrist than Warren, the two appear to be fighting over a similar demographic niche - highly educated white voters.

It's far too early to count Warren out.

She has a strong operation on the ground in Iowa and a good showing there - a win is by no means out of the question - could catapult her into New Hampshire, which adjoins her Massachusetts base. She is also formidable on the stump.

Warren is in a battle with Sanders to become the standard-bearer of the left. If she prevails, she could easily win the nomination.

4. South Bend, Ind., Mayor Pete Buttigieg (July ranking: 5)

Buttigieg has been the one big surprise of the campaign so far.

The 37-year-old mayor of a medium-sized Midwestern city has become one of the Big Four contesting the nomination, even as high-profile senators such as Harris and Sen. Kirsten Gillibrand (D-N.Y.) have exited.

Buttigieg's appeal is built partly on his erudite media appearances. He offers Democratic voters a similar brand of centrism to Biden but in a more youthful, vigorous vessel.

Buttigieg is seeking to become the first openly gay nominee of a major party, which is a draw for some social progressives.

Buttigieg is currently topping the polls in Iowa. A win there could propel him in other states.

The mayor has struggled badly to win any significant black support at all, however. In national polls, he is in fourth place.

That doesn't make his path to the nomination impossible, but it is a steeper uphill climb for him than for Biden, Sanders or Warren.

5. Sen. Amy Klobuchar (D-Minn.) (July ranking: 9)

The Minnesota senator's chances rest upon her outperforming in Iowa.

She is polling at around 6 percent there, which is better than her showing nationally or in other early states.

Klobuchar has been an effective presence on debate stages. But she is battling both Biden and Buttigieg in the centrist lane and has struggled to gain traction.

She could also run into money difficulties. Her cash-in-hand total of $3.7 million at the end of the third quarter was about one-tenth of Sanders's total and put her in eighth place in the money race.

Klobuchar likely needs either Biden or Buttigieg to stumble badly if she is to get into real contention.

6. Former New York City Mayor Michael Bloomberg (July ranking: n/a)

Bloomberg entered the race in late November. He immediately ran into a headwind from critics alleging he was trying to buy the nomination.

The former New York City Mayor has an estimated net worth of around $54 billion, making him one of the 20 richest people in the world.

Bloomberg has already spent lavishly on campaign ads. They have lifted him to fifth place in the national polling averages of both RCP and FiveThirtyEight.

Bloomberg plans to stay out of the early primaries, instead choosing to come into the race at the relatively late stage of Super Tuesday on March 3.

That tactic seems fraught with danger, and it is far from clear that the Democratic grassroots are really seeking a billionaire technocrat who for years supported the controversial police practice known as stop and frisk.

Bloomberg recanted that support right before joining the race, but it is still hard to see a realistic path to the nomination for him.

7. Businessman Andrew Yang (July ranking: unranked)

Yang has carved out a niche as an outside-the-box thinker, and he has drawn enough of a following - the self-proclaimed "Yang Gang" - to surpass expectations.

It's virtually inconceivable that Yang could become the nominee, but he has boosted his national profile and has performed creditably at debates.

Yang would likely be too idiosyncratic to be anyone's vice presidential running mate, but he has built himself a solid platform to stay in public life.

8. Sen. Cory Booker (D-N.J.) (July ranking: 7)

The campaign has never quite come together for Booker, whose candidacy was talked up in its early stages.

He is mired below 3 percent in the national polling averages and is not showing obvious strength in any of the early states. The exit of Harris, the only other major black candidate, has not helped Booker's standing.

At this stage, it seems probable that Booker will stay in the race at least until Iowa. But the rationale for an extended run is questionable when he has been so thoroughly eclipsed by his rivals.

9. Rep. Tulsi Gabbard (D-Hawaii) (July ranking: 10)

Gabbard is not going to be the nominee. Her iconoclastic stances, ranging from her 2017 visit to Syrian leader Bashar Assad to her votes of "present" on Trump's impeachment, guarantee that.

But Gabbard has a niche among a certain kind of unconventional leftist. In New Hampshire, she is in fifth place in the RCP average, with almost 6 percent support.

Speculation about a third-party run by Gabbard has not faded, despite her denials that she is contemplating such a move.

10. Businessman Tom Steyer (July ranking: n/a)

Steyer, a billionaire and a longtime campaigner for environmental causes, has spent heavily in pursuit of his presidential ambitions.

That's been enough to lift him above other seasoned politicians in the polls, but he's a long way from being a genuine contender for the nomination.

Other Candidates:

Sen. Michael Bennet (D-Colo.), former Housing and Urban Development Secretary Julián Castro, former Rep. John Delaney (D-Md.), former Massachusetts Gov. Deval Patrick (D), and author Marianne Williamson.