After a strong stretch for Democratic presidential front-runner Hillary Rodham Clinton, Sen. Bernie Sanders has launched a new phase of his campaign: going after her.

October has been very good to Clinton, who dominated the first Democratic debate and emerged unscathed from a pressure-laden congressional hearing on the terrorist attacks in Benghazi, Libya. She has built a formidable organization, notably in Iowa, where the first votes of the election cycle will be cast in the state’s Feb. 1 caucuses.

Aware of the new landscape, Sanders (I-Vt.) is drawing new and more aggressive contrasts with Clinton.

In an e-mail blast sent to supporters Friday, a day after the hearing on Benghazi, Sanders presented a timeline of his decades-old stances on issues such as gay rights, trade and Wall Street regulation. The e-mail makes clear that, in every area, his record is in line with most grass-roots Democratic voters — and that he has held his positions for far longer than Clinton has.

At a high-profile dinner in Iowa on Saturday, Sanders continued his effort to draw distinctions, noting his early opposition to the Keystone XL oil pipeline, a stance that Clinton only recently adopted. Sanders told the 6,000 die-hard Democrats in attendance that he considered the issue a “no-brainer.”

Sanders also took his new act to the airwaves Sunday morning. “We have differences of opinion, and I think the American people, people participating in the Democratic primary process, need to know the differences,” he said during an appearance on CNN’s “State of the Union.”

Expect more of that to come. With Vice President Biden’s decision not to run for president and the departure of two other candidates over the past week, the race for the Democratic nomination has shaped into essentially a two-way match-up between Clinton and Sanders.

Sanders’s challenge is to prevent it from becoming a lopsided two-way race.

In Iowa, the self-described democratic socialist took aim at the Trans-Pacific Partnership, the trade pact among Pacific nations that Clinton once called “the gold standard” of agreements but has recently opposed.

“It is not now, nor has it ever been, the gold standard of trade agreements,” Sanders said. “I did not support it yesterday. I do not support it today. And I will not support it tomorrow.”

Sanders did not mention Clinton by name in his remarks at Saturday night’s Jefferson-Jackson Dinner in Des Moines or in the e-mail blast. But he and his advisers freely acknowledge that the campaign is looking for ways to draw contrasts with the former secretary of state. And the exercise could accelerate after Sanders starts airing his first TV ads next month.

Former president Bill Clinton speaks to Vermont Sen. Bernie Sanders and his wife, Jane Sanders, at the Jefferson-Jackson Dinner in Des Moines, Iowa, Saturday. (Daniel Acker/Bloomberg)

“We want people to look at his ideas, look at her ideas, look at their track records,” said Tad Devine, a veteran Democratic consultant who is advising Sanders. “In the end, it’s about credibility.”

Among the other differences the Sanders camp sees: Clinton opposed same-sex marriage while she was a senator, she voted to authorize the invasion of Iraq and she has not embraced a proposal to separate commercial and investment banking that Sanders favors.

To underscore his long-standing views, the Friday e-mail includes a series of pictures dating back to a black-and-white one from his days as a college student — each marking a stand he took that seemed ahead of its time. In the 1960s, he led a sit-in in Chicago to protest segregated student housing. As mayor of Burlington in 1983, he signed a proclamation for a gay and lesbian pride day. During his first term in Congress in 1991, he voted to oppose the federal death penalty, and two years later, he voted against the North American Free Trade Agreement. Other positions highlighted include votes against the deregulation of Wall Street, against the Patriot Act and against the Iraq war.

“A presidential campaign should be about the issues,” the e-mail states. “Bernie’s been right about the issues, early and often.”

The maneuvering by Sanders comes amid what has been one of Clinton’s best stretches of the campaign. In addition to her debate and hearing performances, Biden’s announcement that he would forgo a White House bid has largely favored Clinton, at least in the short term, according to fresh polls.

With two others dropping out of the Democratic field this week — former Virginia senator Jim Webb and former Rhode Island governor Lincoln Chafee — the field finally appears settled. The only other Democrat who remains is former Maryland governor Martin O’Malley, who lags far behind his two rivals.

Joe Trippi, a longtime Democratic strategist not working for any of the campaigns this cycle, said a head-to-head contest exposes a weakness for Sanders that has dogged him since the start of the campaign. Although Sanders generates great enthusiasm among white liberals — as evidenced by the thousands of people he draws to his rallies — he has not yet demonstrated he can broaden his appeal beyond that group.

With multiple candidates dividing up the Democratic vote, Sanders’s base might be large enough to win some states, Trippi said, but that becomes harder in a two-way matchup.

“Sanders should have been leading the ‘Run Joe Run’ movement,” Trippi said. “Now he has to hope that O’Malley gets energized and draws some votes from Clinton.”

In recent days, a lead that Sanders enjoyed in New Hampshire has disappeared. The race there is now a dead heat, according to an average of five recent polls by Real Clear Politics.

Meanwhile, in Iowa, a pair of recent polls without Biden in the mix showed Clinton’s once-modest lead there growing by between 7 and 11 percentage points.

Other events have also added to Clinton’s momentum, including a highly sought endorsement Friday from the American Federation of State, County and Municipal Employees, a massive public-sector union.

Sanders’s team has played down any concerns, arguing that the candidate is still in the process of introducing himself and has proved to be a prodigious fundraiser. While Clinton has been airing TV ads in Iowa and New Hampshire since early August, Sanders has yet to hit the airwaves.

“They’ve spent $5 million on TV in Iowa and New Hampshire,” said Jeff Weaver, Sanders’s campaign manager. “We haven’t spent a penny. Given that, we’re doing extremely well. This campaign is going to be competitive. We’re going to have the resources to go all the way to the convention.”

Sanders’s advisers wouldn’t say much about what their television advertising will look like, though it’s likely to begin with biographical spots about Sanders.

The 74-year-old senator, whose career in elected office dates to the early 1980s, often boasts that he has never run a negative ad and says he doesn’t intend to start in the Democratic primary. His advisers say that personal attacks are out of bounds — “we’re not going to try to knife her up out there,” Devine said — but that highlighting issue differences is fair game.

The Clinton camp still has its eye on Sanders, too — and thinks she has several issues that cut in her favor, none more than gun control. Sanders, who represents a largely rural state with a proud hunting tradition, has a mixed record on the issue, including a vote against the landmark 1993 Brady bill.

At a gathering of Democratic women Friday in Washington that both candidates attended, Clinton dwelled on her views on guns, saying she wouldn’t be silenced by the gun lobby.

“Stopping gun violence is worth fighting for, and I’m ready to go,” Clinton told the crowd of about 600 people. It was an issue she also highlighted Saturday night in Des Moines.

Sanders is not shying away from what he sees as another advantage of his candidacy: an ability to attract new voters to the polls.

“Establishment politics, the same ol’ same ol’, ain’t gonna do it,” said Sanders. To win elections, he said, “you rally millions of working-class people who have given up on the political process. You rally young people who have given up on the political process.”