When Bernie Sanders entered the presidential race last spring, he was considered a fringe figure — an unapologetic independent whose “political revolution” would eschew campaign customs and center around his liberal agenda.

Now, with an upset over Hillary Clinton in next Monday’s Iowa caucuses potentially within his grasp, Sanders has emerged as a more combative — and in some ways, more conventionally political — candidate.

Sanders opens his rallies by ticking through the latest polls — an uncharacteristic touch of bravado intended to convince Democrats that he is not only viable in a general election but a stronger standard-bearer against the Republicans than Clinton.

He also is attacking Clinton more directly, not only on policy differences but also on personal character, demonstrating that he has both the stomach and the punch for a political brawl — even one against the Clintons and their defenders.

The Sanders pivot was evident in an interview with The Washington Post on Saturday as he flew on a chartered jet — itself a change for a candidate who used to fold himself into the middle seats of Southwest Airlines planes — from his home in Burlington, Vt., to Iowa for his final week of campaigning here.

Bernie Sanders regularly calls for a "political revolution" in America, but what does that mean? (Peter Stevenson/The Washington Post)

Asked about his comments last spring that he had no intention of playing the role of spoiler or weakening Clinton’s standing in the general election, Sanders turned the question on its head.

“That’s a two-way question, isn’t it?” he said. “When Hillary Clinton’s hit man is throwing garbage at the media, she is in a sense making it harder for me to win the general election.

“Our campaign is not going to simply sit back and accept all of these attacks,” Sanders added. “We are going to win this thing.”

In another sign of growing confidence, Sanders has stepped up his talk of the general election. “I would very much look forward to a race against Donald Trump,” he said Sunday on NBC’s “Meet the Press.” In speeches at his rallies, he sprinkles in previews of “a Sanders administration.”

Over the course of The Post interview, Sanders said Clinton was running a “desperate” campaign incapable of generating the kind of excitement his has. He raised questions about her motives and character. He said he expects Clinton and her campaign to “throw the kitchen sink” at him in the coming week in what he described as a craven attempt to avoid an embarrassing loss in Iowa.

Sanders questioned Clinton’s association with David Brock, the head of the pro-Clinton super PAC Correct the Record, whom Sanders called a “hit man.”

Supporters of Democratic presidential candidate Bernie Sanders listen as he speaks at a town hall campaign event at Luther College in Decorah, Iowa, on Sunday. (Mark Kauzlarich/Reuters)

In recent media interviews, Brock has questioned Sanders’s commitment to African Americans and derisively labeled the senator, who self-identifies as a democratic socialist, as “a socialist.” Brock also reportedly planned to make an issue of the 74-year-old Sanders’s fitness for office — and demand that he release his health records. Brock begged off after Clinton campaign chairman John Podesta tweeted at Brock to “chill out.”

“As somebody who respects Secretary Clinton, its saddens me that she would go to a professional political hit man,” Sanders told The Post.

He recalled Brock’s efforts 25 years ago as a conservative journalist to “destroy” Anita Hill, after she accused then-Supreme Court nominee Clarence Thomas of sexual harassment. “Why do you have to go to somebody like that to run your super PAC?” Sanders said. “There are honorable people out there.”

The intensified scrutiny of Sanders by Clinton and her allies — on abortion rights, health reform and gun control — has had at least part of its desired effect, however, by putting Sanders onto the defensive.

After days of Clinton attacks over his 2005 Senate vote effectively giving immunity from liability suits to gun manufacturers, Sanders reversed his position. Clinton also was relentless in her critique of Sanders’s single-payer, Medicare-for-all health plan because it would have been run by the states, many of which are led by Republican governors. Just hours before their last debate, on Jan. 17, Sanders released a new health plan that would be administered federally.

Last week, after criticism from Clinton, Sanders felt compelled to walk back his comments calling Planned Parenthood and the Human Rights Campaign, which have endorsed Clinton, part of the political “establishment.”

Even as he tries to claim the moral high ground, Sanders is stepping up his critique of Clinton considerably. For months, he has drawn sharp contrasts with her over issues, and he vowed never to go after her personally or with attack ads.

But at recent campaign stops, Sanders has decried Clinton’s acceptance of hundreds of thousands of dollars in speaking fees from banking and corporate interests in the run-up to her 2016 campaign. He singles out her payments from the giant investment firm Goldman Sachs.

“I do not believe that you can get huge speaking fees from Goldman Sachs and then with a straight face tell the American people that you’re prepared to do what is necessary to take on the greed and illegal behavior on Wall Street,” Sanders said. “I don’t think people think that passes the laugh test. . . . Why do special powerful interests give you money? Are they dumb? I don’t think so.”

Recent polls show Sanders pulling even with Clinton in Iowa and outpacing her in New Hampshire, next door to his home state.

Sanders’s crowds are swelling. On Sunday, he drew a boisterous audience of 2,200 to a gym at Luther College here in the northeastern corner of Iowa. Sanders said the grass-roots enthusiasm on display at his events positions him to contend for the nomination over the long haul.

“What this campaign is about, and I’m seeing it every day, is an excitement and energy that does not exist and will not exist in the Clinton campaign,” Sanders said in The Post interview. “We have the capability to have a very good voter turnout. When we have a very good voter turnout, we retain the White House, we regain the Senate, we do well in the House, and we win statewide elections.”

Sanders has taken to starting his rallies by touting polls that show him with larger leads against Trump and other leading Republicans than Clinton. He is trying to argue that he is more likely than Clinton to replicate the kind of general election enthusiasm that propelled Barack Obama to the White House in 2008.

Clinton says that she — not Sanders — is the Democrat whom Republicans fear the most. She cites recent attacks on her from Republicans, including from American Crossroads, the super PAC run by Karl Rove, the former George W. Bush strategist.

“I’ve got to tell you, this is perversely flattering to have Karl Rove go collect money from the financial industry to start running ads against me, to try to convince Democrats not to support me,” Clinton said Saturday in Iowa.

Her allies, including a battery of party leaders, have gone further, arguing that Sanders atop the party’s ticket would be catastrophic.

Clinton is trying to make the case that Sanders lacks the experience, judgment and practicality to be an effective president. In that, Sanders said he sees a parallel to 2008.

He said in the interview with The Post, “If you look at the arguments they’re raising against me — don’t have enough experience in foreign policy, I’m a pie-in-the-sky kind of guy and promising things that are unrealistic — I think if you check it out, that’s very much what they said about Barack Obama in 2008.”

There are indications beyond his message and strategy that Sanders is assuming the trappings of a more traditional politician. He rides around Iowa these days on a hulking, luxurious bus. His avalanche of low-dollar fundraising has enabled him to roughly match Clinton in television advertising. A few months ago, he begrudgingly hired a pollster. And most of his campaign stops are set up by a professional advance staff, orchestrated to provide picturesque angles for the national TV cameras that now follow him.

Sanders has tweaked his appearance, too. That frizzy pile of white hair? He got it cut recently, apparently to look more presidential. Sanders joked at a recent campaign stop that he did so at the insistence of his wife, Jane.

“Enough is enough,” he recalled her telling him.

Rucker reported from Des Moines.