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.

Today’s trio of political storylines

Today’s political news features three different events, in three different cities, with three different pairs of actors. The first storyline (in chronological order) takes place at 11:00 am ET in Portsmouth, NH, where Bernie Sanders is set to endorse Hillary Clinton 36 days after she became her party’s presumptive presidential nominee. “The event will help fuse two wings of the Democratic party together that, despite their real differences, seem to be closer together than their counterparts across the aisle. Many of Trump's primary challengers have yet to endorse him,” NBC’s Alex Seitz-Wald says. Today’s second event occurs at 1:40 pm ET in Dallas, TX, where President Obama and former President George W. Bush attend an interfaith memorial service honoring the five police officers killed last Thursday night. As NBC’s Gabe Gutierrez mentioned on “Today” this morning, it’s the 11th time of his presidency that Obama has memorialized the victims of gun violence. (First Lady Michelle Obama, former First Lady Laura Bush, Vice President Joe Biden, and Dr. Jill Biden also will be in attendance.) And the third and final event takes place at 7:15 pm ET in Westfield, IN (outside of Indianapolis), where Donald Trump stumps with Indiana Gov. Mike Pence, who’s one of the frontrunners -- if not the main frontrunner -- to be Trump’s VP running mate. It’s Trump third VP tryout, after previously appearing with Newt Gingrich (last week) and Chris Christie (yesterday).

The unity factor

A tale of two parties: As for Hillary Clinton’s event with Bernie Sanders in New Hampshire, we’ll be watching to see how enthusiastic -- and genuine -- Sanders’ expected endorsement will be after their hard-fought campaign. (Indeed, the Republican National Committee is out this morning with what it says are Sanders’ Top 15 attacks on Clinton.) But one thing is for sure no matter how far Sanders’ endorsement goes: The Democratic Party is significantly more united heading into November’s general election than the GOP is. Consider the former Republican presidents and presidential nominees who are sitting on the sidelines (the Bush clan, Mitt Romney); or Trump’s GOP rivals who still haven’t endorsed him (Ted Cruz, John Kasich, Jeb Bush); or next week’s GOP convention in Cleveland, where Republican delegates are trying to dump Trump as their nominee. Back in May, we wrote that the political party that’s more divided typically loses. Think back to 2000, when Al Gore was distancing himself from Bill Clinton. Or 2008, when Republicans were running away from George W. Bush and his low approval ratings. Or 2014, when Senate candidates like Alison Grimes in Kentucky were refusing to say if they had voted for Obama. Though we have to ask: If not for Trump, would Democrats be this united (and Republicans this divided)?

A missed opportunity for Obama?

As for Obama’s travel to Dallas today, was it a missed opportunity that today also didn’t include trips to Baton Rouge, LA and St. Paul, MN, where last week’s other violence took place? Think about if he hit all three places in one day…

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NBC|SurveyMonkey tracking poll: Clinton’s lead shrinks to three points, 47%-44%

“Hillary Clinton's lead over Donald Trump narrowed to three points this week after several days of controversy following FBI Director James Comey's recommendation that no criminal charges be brought against the former secretary of state over her use of a private email server,” NBC News writes of the latest NBC|SurveyMonkey tracking poll, which last week had Clinton with a five-point advantage, 48%-43%. But do note that this change is statistically insignificant.

Veepstakes Watch

Mike Pence confirmed yesterday that he’ll attend -- and maybe speak at -- Trump’s rally today… Just who is Mike Pence? We profiled him yesterday… Possible pick Michael Flynn now says abortion is a “legal issue”… The New York Times notes that the GOP’s brightest stars aren’t in the mix for Trump’s VP… Sean Hannity says “I wouldn't be happy with anyone but Newt”… Gingrich told NBC’s Shaquille Brewster he expects the Trump campaign will “let everybody know probably Thursday or Friday”… Kelly O’Donnell confirms that Chris Christie met with A.B. Culvahouse and lasted for about three hours… Tim Kaine said on Meet the Press Daily “In my gut right now, I kinda feel like I'm going to stay in the Senate”… Asked how he’d respond if he was asked to be the pick, Sherrod Brown said “I don’t think she will. I’m not going to speculate”… And Elizabeth Warren is pushing for Republicans to confirm judicial nominees.

Bayh, Bayh GOP control of the Senate?

A few weeks after Mitch McConnell pulled a rabbit out of his hat to convince Marco Rubio to run for Senate re-election, Harry Reid and Chuck Schumer one-upped him -- by getting former Sen. Evan Bayh to run for his old Senate seat (and getting the current Dem nominee, Baron Hill, to withdraw his candidacy). The development now gives Democrats seven GOP-held seats in which they have a 50%-50% chance (or greater) of winning:

Illinois: Mark Kirk (R) vs. Tammy Duckworth (D)

Wisconsin: Ron Johnson (R) vs. Russ Feingold (D)

Pennsylvania: Pat Toomey (R) vs. Katie McGinty (D)

Ohio: Rob Portman (R) vs. Ted Strickland (D)

New Hampshire: Kelly Ayotte vs. Maggie Hassan (D)

Florida: Marco Rubio vs. Patrick Murphy or Alan Grayson (D)

Indiana: Todd Young (R) vs. Evan Bayh (D)

By contrast, Republicans really have one 50%-50% (or better) pickup -- in Nevada. To win back control of the Senate, Democrats need to pick up a net gain of four Senate seats if Clinton wins in November (because her vice president can cast the tie-breaking vote). If Trump wins, Democrats will need to win a net gain of five seats. So yesterday’s news is a big deal: It gives Democrats an additional pickup opportunity. And we’re not even talking about the Senate races where Dems have legitimate -- but less than 50%-50% -- shots (in North Carolina, Arizona, Iowa, and Missouri). But do remember: These Senate comebacks don’t always turn out as planned. Just ask Bob Kerrey.

On the trail

Hillary Clinton campaigns with Bernie Sanders in Portsmouth, NH at 11:00 am ET… And Donald Trump stumps with Mike Pence in Westfield, IN at 7:15 pm ET. 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: 2 days

Countdown to GOP convention: 6 days

Countdown to Dem convention: 13 days