Hillary Clinton and Bernie Sanders at a recent presidential candidates debate. (Mic Smith/AP)

A lot’s happened since Democratic presidential hopefuls last met on a debate stage in mid-January.

Hillary Clinton scraped out a win in Iowa on Monday. Before the voting was even finished, her distant challenger, former Maryland governor Martin O’Malley (D) ended his presidential campaign. The Democratic Party and the campaigns have agreed to add four new, sanctioned debates to the schedule, one of which is Thursday in New Hampshire.

And the Democratic primary is now a showdown between just two candidates: Sen. Bernie Sanders of Vermont and Clinton.

Polls show Sanders ahead of Clinton by almost 18 percentage points in the next voting contest, Tuesday’s New Hampshire primary. Clinton’s team plays this down, saying Sanders has the equivalent of home-state advantage. But it’s undeniable that with just two candidates in the race, things are heating up, thanks in part to Sanders ramping up his attacks. A lot of the top issues in Thursday’s debate are attacks he instigated recently.

Here are the top seven issues.

1. Who’s more progressive?
Democratic presidential candidate Hillary Clinton defends her record as a progressive after her rival, Bernie Sanders, said she was a progressive on “some days.” (Reuters)

Hillary Clinton supported free trade when she was secretary of state. She’s a fan of nation-building, as the United States and its allies attempted in Libya. She wavered on the Keystone XL pipeline before coming out against it.

Those are Sanders’s core arguments for why, in his words, Clinton is a progressive except when she’s a moderate. His campaign sent out a series of tweets Wednesday with this theme:

Clinton has called that a “low blow,” pointing to her support for equal pay for women, same-sex marriage and comprehensive immigration reform. Not to mention she’s arguably to the left of Sanders on gun control.

And Clinton has argued that Sanders’s leftward policies, like universal health care and free public college, are unattainable in today’s divided government.

But the charges seemed to hit home when Sanders repeated them at a Wednesday town hall in New Hampshire hosted by CNN, putting Clinton on the defensive.

2. Clinton’s ties to Wall Street

Perhaps Sanders’s most hard-hitting argument for why Clinton is not a true progressive is this:

The implication is that Clinton has ties with Wall Street going back to her days as New York senator and is thus not fit to lead the “political revolution” that Sanders says is necessary to fix the nation’s income inequality.

This campaign, Clinton has amped up her populist rhetoric to try to avoid those criticisms. She’s called for more rules and enforcement to avoid another financial crisis, among other proposals. And she’s argued she’s taken more money from teachers than bankers.

But The Washington Post’s campaign finance team reports that $1 of every $10 dollars contributed to Clinton’s campaign has come from financial institutions like big banks.

Her collection of multi-hundred-thousand-dollar speaking fees from some of these institutions is another potential sore spot. “That’s what they offered,” Clinton said at Wednesday’s town hall in response to CNN’s Anderson Cooper’s question about her decision to accept $675,000 in speaking fees from Goldman Sachs over a period of a few years.

Expect that to come up Thursday.

3. Hillary Clinton’s emails

Drip, drip drip. Throughout this campaign, there’s been a steady stream of news about Clinton’s exclusive use of a private email server while secretary of state.

An FBI investigation into the emails (not Clinton specifically) is ongoing.

Clinton has maintained she did nothing wrong and did not handle classified material on her server. But in the State Department’s most recent court-ordered release of her emails, officials said seven email chains contained “top secret” information and wouldn’t be released.

It’s yet another issue that’s put Clinton on the defensive.

She’s gone from saying no classified emails passed her server to saying that no emails that were marked classified were sent or received on her private sever. The Washington Post Fact Checker team reported Clinton actually has the power to decide what State Department material is classified, though that doesn’t extend to material from other agencies like the CIA.

After saying in an October debate that “the American people are sick and tired of hearing about your damn emails,” Sanders has changed his tone, saying the potential that classified information was on her sever is a serious issue, but adding he’ll let the American people decide what was right or wrong.

4. National security
Former secretary of state and Democratic presidential hopeful Hillary Clinton outlined her national security plan against the Islamic State in the wake of the attack in San Bernadino, Calif. She spoke at the University of Minnesota in Minneapolis. (Reuters)

This is Clinton’s strong suit, and it’s a topic she has dominated in the past four debates.

Clinton is the most trusted 2016 candidate on terrorism, according to a November Washington Post poll.

She often pivots to a general election focus when talking about what she’d do, attempting to balance how to come across as hawkish as possible with a base that is increasingly worried about terrorism — but also wary of military intervention in the Middle East.

Washington Post-ABC News November poll

There’s some tension with her former boss, who Americans give low marks on terrorism. When Democrats gathered for their November debate, a day removed from the Paris attacks, Clinton not-so-subtly rebuked President Obama’s comments that the Islamic State had been “contained.”

Sanders has struggled to match Clinton’s intensity on national security and terrorism, preferring to talk about his bread-and-butter issue of economic inequality. He  tried to get a leg up in December’s ABC debate by questioning Clinton’s decision to support regime change in the Middle East while she was secretary of state.

5. Gun control

Polls consistently show that as many as 90 percent of Americans support at least expanding background checks, but a majority of Americans are doubtful that changing gun laws will help prevent gun violence.

In other words, it’s complicated.

Clinton has staked out a position on gun control to the left of her 2008 presidential campaign;  she recently called for a renewal of the federal ban on so-called assault weapons. She’s been campaigning in New Hampshire with former U.S. representative Gabrielle Giffords (D-Ariz.), who was shot and almost killed at a public meeting in 2011 and now advocates for gun control.

Addressing gun control proved to be a struggle for Sanders in the first debate, as Clinton attacked him for a voting record she said looks more like an NRA supporter. Sanders says he was representing his rural, Vermont community, which remains very pro-gun rights. He specifically came under fire for voting for a 2005 law that gives gun manufacturers legal immunity from damages their guns might cause.

A day before January’s debate, Sanders said he would be open to a bill introduced in the Senate to reverse some key parts of that law. “As I have said for many months now, we need to look at the underlying law and tighten it up,” Sanders said in a statement.

But how much the candidates can do on gun safety is an open question. Congress has failed to pass any gun control measures. A proposal to ban terrorism suspects on a secretive no-fly list from being able to buy guns — and one to expand background checks — failed in the Senate one day after the San Bernardino, Calif., shooting.

6. The Flint water crisis
The House Oversight and Government Reform committee held an outrage-filled hearing on Feb. 3 about the drinking water crisis in Flint, Mich. (C-SPAN)

Toward the end of Democrats’ January debate, Clinton turned suddenly toward the Flint water crisis. She had sent a top campaign aide to the hardscrabble town about 70 miles outside Detroit to see how residents were coping with being pumped poisonous water for nearly 18 months.

“I want to be a president who takes care of the big problems and the problems that are affecting the people of our country every day,” she said, seizing the political moment.

Sanders called on Michigan Gov. Rick Snyder (R) to resign: “A man who acts that irresponsibly should not be in power.”

Since then, Flint has continued to dominate national headlines. Snyder has apologized twice and watched his approval ratings plummet, although a majority of residents don’t think he should step down. State and federal officials testified Wednesday in Congress in an emotional hearing. And Senate Democrats blocked an energy policy bill Thursday after Republicans declined to provide hundreds of millions of dollars of aid for Flint.

The issue has large racial overtones; Flint is mostly poor and mostly black. Clinton has high approval ratings among black voters; support among non-white voters is one of Sanders’s main weaknesses.

(Philip Bump/The Washington Post)
7. Climate change and energy
Delegates from 196 nations approved a historic climate deal after 13 days of negotiating on Saturday, Dec. 12. Here’s what you need to know about the accord. (Monica Akhtar/The Washington Post)

Democratic presidential candidates agree the government needs to step in to help stop climate change and invest in more clean energy, and all the candidates cheered a historic, 196-country agreement reached in Paris to try to limit the Earth’s warming by 2 degrees Celsius above pre-industrial levels.

The litmus test among environmentalists has long been whether candidates would approve the fourth stage of the Keystone pipeline to ship Canadian oil to Nebraska, a politically touchy subject for Clinton who recently said she opposes the pipeline but not before wavering for months.

Obama took that test off the table in November when he announced he wouldn’t approve the pipeline.

Clinton has now shifted her focus to trying to invest in coal communities hit by a shift to clean energy: Before Democrats’ November debate, she unveiled a $30 billion infrastructure and tax-break plan to do just that.

Sanders often talks about how climate change is one of the United States’ most pressing problems, as well.