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.

Mitch McConnell’s Supreme Court dilemma

With yet another poll showing Hillary Clinton ahead of Donald Trump by double digits — this time from NBC|SurveyMonkey — Senate Majority Leader Mitch McConnell has an important choice to make. Does he continue to block President Obama’s pick to fill the Supreme Court vacancy, Merrick Garland, and risk the possibility that a President Hillary Clinton could nominate someone much more liberal (and younger) instead? Or does he relent on the Garland blockade, realizing that it might be the best outcome for Senate Republicans — simply to turn the conversation away from Trump?

McConnell’s office tells us that he remains firm in his opposition to the Garland pick. “The Leader has been clear: The next president will make the nomination for this vacancy,” says Deputy Chief of Staff Don Stewart. But Democrats are making a separate argument: If Republicans are looking for any way to separate themselves from Trump, moving on Garland would do the trick. “Congress is likely to be in session for a grand total of 20 days between now and the election, and it's clear that confirming Garland a vote is the only concrete, news-driving step that Republicans can take to separate themselves from Trump,” a top Democratic Senate official says.

Kaine doesn’t close the door on possibility that Clinton could make a different SCOTUS pick

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And it’s worth pointing out that Clinton running mate Tim Kaine didn’t 100% close the door on the possibility that Clinton might make her own Supreme Court pick if Republicans continue to block Garland. Here’s what Kaine said on NBC’s “Meet the Press” last Sunday:

CHUCK TODD: Do you believe that Merrick Garland's nomination should be taken up during the lame duck, regardless of whether you guys win or lose?KAINE: Absolutely.TODD: So if Secretary Clinton wins—KAINE: He is—TODD: —he should be taken up as the nominee?KAINE: This will be for the president and the president-elect to decide. But look, if it comes up for a vote in the Senate, Merrick Garland gets so far over the hurdle of the fitness and character test that is supposed to be the legitimate question for nominees, of course I'd vote for him.TODD: So it's not 100 percent, though, that his nomination stays if your ticket wins?KAINE: You know, I think the Dems may well take the Senate. In fact, I think we're going to. But it will be the Republican majority that will be running the floor until the next Senate comes in place. I have no idea whether they will allow the nomination to be taken up. They pledged that they won't. But if it does, I'm going to vote for him.

NBC|SurveyMonkey poll: Clinton 51%, Trump 41%

As for that new NBC|SurveyMonkey online tracking poll we mentioned above... “Hillary Clinton now holds a 10-point lead over Donald Trump, 51 percent to 41 percent, according to results from the latest NBC News|SurveyMonkey Weekly Election Tracking poll Clinton's double-digit lead is up from an 8-point gap last week. The 10-point margin is the biggest spread in the tracking poll since the two-way general election match-up was first compared in early May.”

Another GOP defection from Trump

Meanwhile, Sen. Susan Collins (R-ME) became the latest Republican to say, unequivocally, that he or she cannot support Trump in the general election. “I will not be voting for Donald Trump for president. This is not a decision I make lightly, for I am a lifelong Republican. But Donald Trump does not reflect historical Republican values nor the inclusive approach to governing that is critical to healing the divisions in our country,” Collins writes in a Washington Post op-ed. “My conclusion about Mr. Trump’s unsuitability for office is based on his disregard for the precept of treating others with respect, an idea that should transcend politics. Instead, he opts to mock the vulnerable and inflame prejudices by attacking ethnic and religious minorities.” And she cites three incidents in particular — Trump mocking the disabled reporter, attacking the federal judge Gonzalo Curiel, and criticizing the Khan family. And this defection comes after 50 former GOP national-security officials signed a letter arguing that Trump “would put at risk our country’s national security and well-being” if he becomes president. We can’t stress enough: These intra-party departures — on this scale — are HIGHLY unusual in our modern presidential politics. You have to go all of the way back to Goldwater in ’64 for a fair comparison.

Paul Ryan’s primary challenge is tonight

Finally, NBC’s Leigh Ann Caldwell writes up House Speaker Paul Ryan’s primary challenge in Wisconsin, which takes place tonight. “What should have been a fairly routine primary challenge for House Speaker Paul Ryan has now become a symbol of Republican Party problems: A fight pitting the establishment against GOP rebels, and their controversial presidential nominee against Washington leadership. Before last week, few people even knew — or cared — that Ryan's primary for re-election was approaching. But when Republican presidential nominee Donald Trump said he wasn't ready to endorse Ryan and then praised his Republican challenger, Paul Nehlen, the political spotlight shined directly on Tuesday's contest.” Make no mistake, Ryan should easily win. But it will be interesting to see by what margin.

On the trail

Hillary Clinton, in Florida, makes a Zika clinic tour in Miami... Donald Trump stumps in North Carolina, holding rallies in Wilmington at 2:00 pm ET and Fayetteville at 6:00 pm ET… Mike Pence is in Pennsylvania… And Tim Kaine is in Austin, TX.

Countdown to Election Day: 91 days