First Read is a morning briefing from Meet the Press and the NBC Political Unit on the day's most important political stories and why they matter.

Why Trump’s flip-flops matter: In the space of hours yesterday, Donald Trump made a complete walk-back of his controversial comments on MSNBC suggesting that he would advocate punishment for women seeking abortions. He went from saying "there has to be some form of punishment, yeah” to "the doctor or any other person performing this illegal act upon a woman would be held legally responsible, not the woman.” Throughout the campaign, instances of Trump changing or amending a statement – or at least enduring withering criticism from within his own party for something he said -- have become routine. On nearly a weekly basis now, downballot Republicans are facing maddening political choices as they’re asked to comment on Trump’s constantly-changing policy positions and shocking pronouncements. And the constant whiplash makes for a compelling case for those inside the party who say it’s worth risking the backlash with Trump voters by doing whatever it takes to stop him. By the fall, can Republicans running downballot really afford to respond almost daily to something Trump said? It’s been tough enough this spring, and it’s only going to get more complicated.

Revisiting the controversies: It’s a particularly apt moment to revisit the cases when Trump has either dramatically changed a controversial position in a short period of time or said something so contentious that condemnations from his own party came as fast and furious as they did yesterday. Here’s a list from an extensive sampling of Trump’s reversals and the comments that drew the most intra-party criticism.

June 2015: On Mexican immigrants: “They’re bringing drugs, they’re bringing crime, they’re rapists.”

GOP critics included: Marco Rubio, Rick Perry and Jeb Bush

July 2015: On John McCain: “I like people who weren’t captured.”

GOP critics included: Scott Walker, Marco Rubio and the Republican National Committee

August 2015: On Megyn Kelly: “There was …. blood coming out of her, wherever.”

GOP critics included: Erick Erickson, Mike Huckabee, Carly Fiorina

August 2015: On health care: "As far as single-payer, it works in Canada.”

(He’s amended his positions on health care numerous times)

GOP critics included Erick Erickson, Rick Perry, Reid Ribble

September 2016: On Carly Fiorina: “Look at that face!”

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(He later said he was referring to Fiorina’s “persona.”)

GOP critics included: Scott Walker, Jeb Bush

November 2015: On 9/11: “Thousands and thousands of people were cheering as that building was coming down.”

GOP critics included: George Pataki, Jeb Bush

December 2015: On Muslims: Enact "a total and complete shutdown of Muslims entering the United States.”

GOP critics included: Reince Priebus, Paul Ryan, Dick Cheney

February 2016: On David Duke’s endorsement: "I know nothing about David Duke.”

GOP critics included: Tim Scott, Mitch McConnell, Paul Ryan

February 2016: On waterboarding: “Torture works.”

GOP critics included: Michael Hayden, Lindsey Graham

March 2016: On Muslims: "I think Islam hates us.”

GOP critics included Marco Rubio, Jeff Flake

March 2016: On military leaders and torture: “When I say they’ll do as I tell them, they’ll do as I tell them.”

(He later clarified to say he acknowledges international laws.)

GOP critics included: A group of 70 Republican foreign policy experts who penned an open letter to Trump on the subject

March 2016: On women seeking an abortion: “There has to be some form of punishment, yeah.”

(He changed his position within hours)

GOP critics included: SBA List, March for Life, Ted Cruz

Marquette poll shows Cruz ahead

Yesterday’s Marquette University Law School poll in Wisconsin showed a pretty significant lead for Ted Cruz in the state, where the Texas senator has gone from trailing Trump badly in February to leading him by 10 points. There are a couple of smart theories out there about why Trump – who could have been tailor-made for a state with a high population of working class and small town conservatives – may be struggling there. As Dave Catanese pointed out yesterday, the same hugely influential conservative radio hosts who helped propel Scott Walker to power have been pummeling Trump, one reason why the frontrunner’s cringe-worthy interview with host Charlie Sykes was so noteworthy. And another smart GOP strategist made this point to us: “One thing about Wisconsin that is unique: There is no civil war in the GOP. Instead, they had a war with the Democrats (over Walker’s budget control act.) It galvanized our side TOGETHER against the bad guys.” If Trump loses big in the Badger State, it will be because the conservative divisions there weren’t deep enough for him to exploit as he has in other places. Also, keep in mind that there is a kind of civility to Wisconsin politics, not unlike there is in states like Iowa and Minnesota – two other states where Trump struggled.

Another big fundraising number for Sanders

NBC’s Kasie Hunt reports this morning that Bernie Sanders has raised close to $40 million so far in March, with the goal of hitting a total of $43.5 million to beat the campaign’s record haul in February. There’s no question that his ability to haul in grassroots cash makes him a powerful threat, even as he trails Clinton in the pledged and overall delegate counts. But it’s also worth noting that Sanders’ fundraising ability that may not ever translate into a boon for the party overall. When our colleague Rachel Maddow asked Sanders in an interview yesterday whether he would consider raising money for House and Senate campaign committees, as Clinton has been doing, Sanders was noncommittal. “Well, we'll see,” he said. “And, I mean right now, again, our focus is on winning the nomination. Secretary Clinton has access to kinds of money, that we don't, that we're not even interested in. So let's take it one step at a time.”

On the trail

Bernie Sanders talks trade in Pittsburgh and holds a rally in the Bronx. Bill and Hillary Clinton both campaign in New York.