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.

How the ’16 campaign became a referendum on Trump

Here’s a thought experiment: Imagine if Marco Rubio, not Donald Trump, were the Republican Party’s presumptive presidential nominee. There would be considerable more attention to the Hillary email story, which still hasn’t gone away. We’d be highlighting how Bernie Sanders still hasn’t quit his race, creating a fissure inside the party. And we’d be fixated on Clinton’s all-time low favorability numbers. Instead, the current stories are Trump attacking a federal judge, how the Republican Party is truly divided over its presumptive nominee (see below for a list of that disunity), and Trump having even worse favorability numbers than Clinton. Bottom line: This 2016 presidential race could have been a referendum on Clinton and the Obama White House, even with the president’s 50%-plus approval rating. Instead, it has turned into a referendum on Trump. As Stu Rothenberg writes, “Donald Trump continues to make the 2016 election a referendum on his accomplishments, his past statements and his beliefs.” And for Republicans, that’s a hard race to win.

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The Unity Gap revisited

So remember yesterday when we told you that Democrats appeared to be uniting in a much more orderly manner than Republicans? Well, consider the events of the last 24 hours. In almost a blink of an eye, President Obama, Elizabeth Warren, and Vice President Joe Biden all endorsed Hillary Clinton, and Warren (a potential VP possibility) meets with Clinton this morning, NBC’s Kasie Hunt confirms. What’s more, it sure sounded like Bernie Sanders -- maybe next week, perhaps the week after -- will join them in support of Clinton. Now compare that with:

George W. Bush, the last GOP president, whose family is sitting out 2016;

Mitt Romney, the last GOP presidential nominee, who is firmly against Trump;

Ted Cruz, who still hasn’t endorsed Trump;

John Kasich, ditto;

House Speaker Paul Ryan, who took almost a month before endorsing;

Senate Majority Leader Mitch McConnell, who said on a Bloomberg podcast that Trump needs to pick an experienced VP “because it's pretty obvious he doesn't know a lot about the issues”;

Sen. Susan Collins (R-ME), who on MSNBC’s “MTP Daily” left open the possibility of backing Clinton;

And Sen. Mark Kirk (R-IL), who this week un-endorsed Trump.

As we wrote a month ago, the party that is less united usually loses. And right now, Democrats smell blood -- one month before the GOP convention in July.

What Bernie Sanders DIDN’T talk about last night

As for Sanders, he held his first campaign rally -- in D.C. (which holds its primary on Tuesday) -- since California, and NBC’s Danny Freeman says it was notable what Sanders DID NOT mention. “Sanders didn’t talk about superdelegates, his own path to the nomination, a contested convention, Philadelphia, and most of all, he didn’t mention Clinton’s name once. He went through certain areas of his stump where he normally hits Clinton, like minimum wage and certain aspects of climate change, but he completely avoided speaking about his opponent. At one point he opened a bit with ‘my opponent’ before correcting himself by saying ‘my opponents and [the Dem establishment]’ so as to not seem as though he was singling Clinton out it seemed.” More from Freeman: “Sanders did not utter a word about his high profile meetings [yesterday] with President Obama, Vice President Biden, Senator Harry Reid, and Senator Chuck Schumer. Nor about Obama’s endorsement of Clinton, or Warren’s impending endorsement. The only thing he mentioned about the state of the race actually was … the fact that vote is still coming in to California due to late ballots mailed in.”

Bernie Sanders speaks to the press outside of the West Wing following a meeting with President Obama. MANDEL NGAN / AFP - Getty Images

Tap the brakes on Warren-palooza

As the VP speculation about Elizabeth Warren heats up, here’s a reality check: Yes, she’s going to get vetted. But unless she and Hillary Clinton click this morning at their meeting -- and keep on clicking -- she isn’t a sure bet to get picked. Why? Because Clinton has already gotten to know the Tim Kaines and Tom Perezes given that they’ve endorsed her. But she doesn’t know Warren (remember, Warren came into the Senate after Clinton left her secretary of state job). What’s more, Warren isn’t necessarily a go-along, get-along person, which is a big requirement of the VP job.

On the trail

Hillary Clinton, in DC, delivers a speech to Planned Parenthood Action Fund at noon ET… And Donald Trump, also in DC, addresses the Faith and Freedom confab at 12:30 pm ET before heading to campaign in Richmond, VA at 8:00 pm ET. On Saturday, Trump hits rallies in Tampa, FL and Moon Township, PA. Don’t forget to check out the political unit’s rolling minute-to-minute coverage of all the latest 2016 developments at the On the Trail liveblog at NBCNews.com. Here’s NBC’s Leigh Ann Caldwell previewing Trump’s Faith and Freedom address.