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 has to pick a conservative VP

We’re exactly a week out from the Republican convention in Cleveland, which means that Donald Trump’s vice-presidential pick could come any day now. And here is the VP reality for Trump: He has to pick a Republican with crimson-red conservative credentials, especially on social issues. Why? Not only are rebellious GOP delegates trying to thwart his nomination at the convention (no matter how unlikely that effort seems); they’re also conspiring to possibly pick Trump’s VP for him, as NBC’s Leigh Ann Caldwell writes. Remember, GOP delegates aren’t bound to vote for Trump’s running mate. For those very reasons, this weekend’s floated VP possibility for Trump -- Lt. Gen. (ret) Michael Flynn -- is most likely a non-starter, given that he’s a registered Democrat and that he supports abortion rights. “These are difficult issues, but I think women have to be able to choose,” Flynn told ABC yesterday. So Trump faces this choice: Pick a conservative, or face a greater chance of a revolt in Cleveland. Someone like Trump, who also once supported abortion rights and who once donated money to Democrats, was always going to have less ideological flexibility in choosing his running mate than, say, Ted Cruz or Marco Rubio. But now it’s even less flexibility with a concerted effort to dump Trump at the convention.

NBC’s three VP frontrunners — Christie, Gingrich, Pence

So given that reality, who are the current frontrunners for Trump’s VP pick? According to NBC’s reporting, there are three names in the top tier (in alphabetical order) -- New Jersey Gov. Chris Christie, former House Speaker Newt Gingrich, and Indiana Gov. Mike Pence. Well, Trump today stumps with Christie in Virginia. And tomorrow, he campaigns in Indiana, where we expect Pence to join him.

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Meanwhile, the Washington Times reports that Pence increasingly looks like the possible choice. “Scrambling among Indiana politicians has reached the point where Republican Party insiders are convinced that presumptive presidential nominee Donald Trump will pick Indiana Gov. Mike Pence as his running mate. Constitutional lawyer James Bopp, an Indiana delegate to the Republican National Convention who is close the governor, told The Washington Times that Indiana House Speaker Brian C. Bosma, 58, a conservative Republican, had sought advice from him on running for governor.” (A reminder: Pence can’t both run for re-election this year, as well as run for VP.) But some caution here: It’s one thing for this news to be the hubbub in Indiana, yet that might not reflect what’s in Trump’s head.

Veepstakes Watch: More on the VP front

NBC’s Andrew Rafferty profiles retired Lt. Gen Michael Flynn, whose name was floated over the weekend as a potential Trump VP pick… New Jersey insiders think Chris Christie is trying to angle his way into the job… Newt Gingrich told the AP Saturday that he is being vetted by “It’s not an automatic Yes.” Gingrich was in Paris over the weekend for a rally of the controversial Iranian dissident group MEK… Mary Fallin said she is not being vetted -- and also characterized Trump as a “racial healer”… POLITICO looks at Hillary Clinton’s relationship with Tom Perez and how their warm rapport is pumping up his chances to be the pick… Cory Booker appeared on Meet the Press… Tim Kaine appeared on Univision for a Spanish-language interview… And check out the latest in the veepstakes with the newest edition of VP Watch…

Sanders set to stump with Clinton Tuesday in New Hampshire

Well, it’s official: Bernie Sanders is set to campaign with Hillary Clinton tomorrow in Portsmouth, NH. And as NBC’s Kristen Welker discussed on “Today” this morning, Sanders is expected to endorse Clinton at the event. Remember, Sanders defeated Clinton in New Hampshire’s February primary by 22 points, 60%-38%. The news of this joint campaign event comes after this weekend’s Democratic platform meeting in Orlando, where Team Sanders won policy concessions on health care and a $15 minimum wage, while they lost on effort to oppose the TPP trade agreement. “Democrats this weekend advanced what is easily their most liberal platform ever as representatives of Bernie Sanders and Hillary Clinton strove toward unity during a sometimes contentious party meeting,” NBC’s Alex Seitz-Wald writes. “The draft platform, which still needs to be ratified at the Democratic National Convention in Philadelphia later this month, showed Sanders' clear influence, even though he lost a battle on his top priority: opposition to the Trans-Pacific Partnership.” A reminder:

Days it took for Clinton to endorse Obama after he became his party’s presumptive nominee: 4 days

Days it’s taken Sanders to endorse Clinton after she became presumptive nominee: 35 days -- and counting (though that’s likely to come to an end tomorrow)

Obama heads to Dallas on Tuesday

As for other big news out there -- the aftermath of the violence in Dallas, Louisiana, and Minnesota -- President Obama is set to appear with former President George W. Bush in Dallas on Tuesday to mourn the death of the slain police officers there… NBC’s Lester Holt will interview Vice President Biden, as Biden meets today with law enforcement leaders to discuss officer training and safety… And on “Meet the Press” yesterday, Homeland Security Secretary Jeh Johnson said, “Let's all dial back the overheated political rhetoric.”

On the trail

Donald Trump campaigns with Chris Christie in Virginia Beach, VA at 1:00 pm ET… The Clinton campaign is up with this web video pre-butting Trump’s national-security-focused event…. And Hillary Clinton fundraises in New York. 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.

Countdown to Rules Committee showdown in Cleveland: 3 days

Countdown to GOP convention: 7 days

Countdown to Dem convention: 14 days