DETROIT - The first thing Bernie Sanders wants you to know about Joe Biden is that "he is my friend."

And "a decent man."

But Sanders wants you to know some other things about Biden, too - that he once supported cuts to Social Security, cast a vote to prohibit federal funding of abortions, and used to favor a ban on openly gay people serving in the military.

In recent speeches, Sen. Sanders, I-Vt., has cautiously tested new and forceful attacks on the former vice president's record on gay rights and women's issues, potent critiques aimed at two key bases of the Democratic Party.

At the same time, Sanders is increasingly assuring crowds - at the beginning, middle and end of speeches - that "I will support Joe Biden" if he becomes the Democratic presidential nominee.

Those two messages highlight the challenge facing Sanders as he campaigns in the Midwest ahead of a crucial primary in Michigan on Tuesday.

To become the nominee, Sanders must beat Biden, a former fellow senator. But at a time when tensions between liberal and moderate Democrats have become heated, Sanders is facing increasing pressure to make sure the party unifies before November, whether he's the nominee or not.

For much of the campaign, Sanders has focused on economics and inequality, attacking his opponents for accepting donations from the wealthy and opposing Medicare-for-all. But in recent days, Sanders has also begun to challenge Biden's record on LGBT rights and women's reproductive health.

He repeatedly knocked Biden for, until recently, supporting the Hyde amendment, which banned Medicaid funds from being used to cover abortions.

He has criticized the former vice president for supporting "don't ask, don't tell" in the 1990s, a policy that prevented openly gay people from serving in the military. And he has stressed Biden's 1996 vote for the Defense of Marriage Act, a federal law that barred legal recognition of same-sex marriages.

"Joe Biden in the past has voted for what is called the Hyde amendment, that said that women could not use Medicaid dollars in order to protect their reproductive rights and get an abortion," Sanders told a crowd of 15,000 supporters Saturday at a rally in Chicago, echoing comments he also made in Phoenix; Detroit; and Flint, Michigan.

"I am proud to tell you that I have a 100% pro-choice voting record throughout my entire life," he added. "I believed then, and I believe now, that it is women who have a right to control their own bodies, not the government."

The new line of attack comes at a crucial moment for the Sanders campaign, which is struggling to gain momentum ahead of Tuesday's votes in six states. Though Sanders racked up wins in some early states, Biden won most of the nominating contests on Super Tuesday amid a consolidation of moderate support and now holds a lead in delegates.

On the stump, Sanders has framed these new comments as a way to emphasize his own liberal record, noting that he supported gay rights when it was far more difficult to do so - unlike Biden. The supercharged contrast on social issues, Sanders said repeatedly in recent days, is crucial to give voters a clear picture of who has "the vision" to lead.

"The point here is not just to look back 20 years ago, not just to look at consistency, it is to look at which candidate had the guts to cast difficult votes because they were the right votes," he said Friday during a news conference. "All I can tell you is, whether it was Iraq, whether it was DOMA, whether it was 'don't ask, don't tell,' those were difficult votes. I was there, on the right side of history, and my friend Joe Biden was not."

The Biden campaign did not return a request for comment about the new attacks from Sanders. In recent days, Biden has largely avoided taking on Sanders directly, instead pitching himself as a "unity candidate."

Sanders has at times looked uncomfortable making these statements. Absent during these sections of his speech are his typical gesticulations and booming crescendo; instead, he has read carefully from his notes, looking down intently, often taking several rhetorical heaves to wind up to his point.

And the attacks always come with a bookend of careful caveats: "Let me once again say, Joe Biden is a friend of mine," he said Friday, during a news conference in Phoenix when asked whether he was worried that the criticism might dampen liberal enthusiasm for Biden if he is the nominee in November. "I think we are the stronger campaign to defeat Donald Trump. You have not heard me say that I think that Biden will not defeat Trump."

The senator's strategy comes with risks.

Already, there are fears that Biden may not be able to energize Sanders' base if he wins the nomination. Many Sanders supporters have expressed uncertainty about whether they will cast ballots in November for anyone else.

Jerome Palmer, 45, said during a rally in Phoenix on Thursday that he identifies as a "progressive" and was until recently a "Democrat with reservations." He said he has struggled with the idea of voting for Biden in November if he is the nominee.

Palmer said he does not want Trump to be reelected, and he knows that he may have to make an uncomfortable political compromise to prevent it. But he cannot shake the "sour taste in my mouth" after casting a ballot for Hillary Clinton in 2016.

The fact that she lost, he said, added insult to injury.

He now identifies as a political independent after a lifetime of voting for Democrats.

"The past four or five years, I've had an awakening. The Democrat Party, they're not the good guys. They're bad news in my opinion," he said, noting that he had always voted for Democrats before. "Democrats, it's literally two parties within one party. I think the moderates are just conservative-lite. It's the progressives who want to see change."

Sanders has never been afraid to go after his Democratic rivals on policy or their posture toward wealth. During the highly contentious 2016 primary fight, Sanders hammered Clinton over the paid speeches she delivered to financial institutions, which netted her tens of millions of dollars.

But he has hesitated to pivot to matters of culture and character. In 2016, he promised publicly never to criticize Clinton over her "damn emails," neutralizing a potent political attack used by her critics concerning her use of a private computer server as secretary of state.

In this cycle, as he has escalated his attacks against Biden, Sanders has sought rhetorical cover by pointing out that the eventual nominee will have to be able to withstand the president's bellicose campaign style. He has stressed that Trump "will be a very formidable opponent for a lot of reasons. We have got to rally our base. And that means rally the women's community, rally the LGBT community."

Democrats have carefully studied the campaign Trump ran in 2016, which regularly featured highly personal attacks. As the Republican primary field began to narrow in March, Trump amplified denigrating comments made on Twitter about Heidi Cruz, the wife of Sen. Ted Cruz, R-Tex.

At one point, he seemed to threaten to release opposition research about her previous battle with depression.

He often called Cruz "Lyin' Ted!" or Sen. Marco Rubio, R-Fla., "Lil' Marco!"

That behavior set the tone for the election, and Trump's rivals in time began to return the barbs, like when Rubio began making jokes about the size of Trump's hands.

Unlike when he is talking about Biden, Sanders' comments on Trump are scathingly personal. He calls him a "pathological liar," a "fraud," a "racist" and an "autocrat." He regularly accuses the president of never having read the Constitution.

By those standards, the final stretch of the Democratic primary may look tame.

Notably, Sanders has not mentioned Biden's son Hunter Biden, whose business associations have already become fodder for Trump's attacks, and were at the center of the congressional inquiry that led to the president's impeachment in January.

Still, the overwhelming bulk of his critiques have focused on health care, prescription drug prices, trade, and campaign finance reform - his bread-and-butter issues.

On those topics, Sanders is loud, aggressive and direct.

“I just don’t think that Joe Biden can generate enthusiasm when you’ve got 60 billionaires contributing to his campaign,” he said Saturday in Dearborn, Michigan, his voice and his hands rising. “At the end of the day, people understand that if you’re taking lots of money for billionaires, you’re not going to be there standing up for the working class and the middle class in this country.”