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.

Flying Blind: What We Know (and Don’t) Heading into Saturday’s Contests

Given the scarcity of good polling, we’re all kind of flying blind as we head into Saturday’s GOP primary in South Carolina and the Democratic caucuses in Nevada. Here’s what we know about South Carolina: First, it’s a tale of two primaries -- Trump vs. Cruz, and the fight for third. Second, for the first time in months (thanks to his debate performance and his brother yesterday), Jeb Bush is a central part of the storyline. And third, like him or not, Donald Trump is still shaping the contours of this GOP race (see Saturday’s debate or yesterday’s Trump press conference).

Here’s what we don’t know. Is Trump falling in the polls after the debate and especially after his tough 9/11 critique on George W. Bush? Or is he in a commanding position to win South Carolina, which would put him in the driver’s seat to win the GOP nomination? And does George W. Bush help Jeb enough to finish third, which would give him a pulse in the Republican race? Or does Marco Rubio (or John Kasich) have a better path to third?

You could make an argument that George W.’s appearance in this GOP contest helps both Trump (who gets to tell it like it is to anti-establishment supporters) and Jeb (who has a final opportunity to rally the establishment). Don’t miss this dispatch from MSNBC’s Benjy Sarlin, who quotes South Carolina Republicans who voted for the Bushes in past primaries but are more than open to Trump’s message.

Reading the Body Language in Nevada

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Turning to Saturday’s Democratic caucuses in Nevada, the body language tells us where the race is despite the lack of a single good public poll out there. Hillary Clinton is nervous (see her additional campaigning in Nevada yesterday), while Bernie Sanders seems like the guy who’s feeling confident (see his campaigning in Michigan, not Nevada, yesterday). As Nevada political expert Jon Ralston told NBC’s Kristen Welker yesterday, “I know Clinton believes the race is in single digits -- I know that for a fact -- and it probably is."

But what does that even mean when you have a caucus, which is going to have relatively low turnout, and which has same-day registration where the Clinton campaign has to be worried about the whole millennial effect? You know, all these young voters deciding to register on Saturday to go support Bernie Sanders?”

Translation: It’s really, really close. But here is something we know about Nevada: This is a race (because of the state’s diversity, because of the momentum it could provide heading into South Carolina) that Sanders NEEDS to win if he’s going to have a good chance at the nomination. A win keeps the pressure on Clinton; a loss gives a Hillary a big boost -- if not a decisive one -- heading into South Carolina and the March 1 races.

Republicans Cede the SCOTUS High Ground to Obama

As for the biggest political story in the land -- the death of Supreme Court Justice Antonin Scalia and the fight over his eventual successor -- we have this observation after the three days of rhetoric since the news first broke: Republicans are giving President Obama the high ground. Why? If they had said, from the outset, that they were willing to give a qualified Obama nominee a fair hearing, they could have used all of their fire to find a single objection to convince their members to vote against the pick.

But by declaring, just hours after Scalia’s death, that any Obama pick -- no matter how qualified -- is unacceptable, they’ve given the president and his Democratic colleagues a gigantic political gift. The question now becomes: What does Obama do with this political gift? Does he nominate someone with moderate/confirmable credentials? Or does he opt for a partisan warrior?

Is This the Fight That Breaks the Senate -- Once and for All?

Maybe more importantly, this political fight over Scalia’s eventual successor could finally break the U.S. Senate, Congress, and all of Washington. Think about: The past congressional fights have been over things like the debt ceiling, the sequester, and the fiscal cliff. But this is the Supreme Court -- something that isn’t as esoteric to most Americans. Ditto the fact that a vacancy could last a year or longer. This is a big deal. It’s maybe not a constitutional crisis yet. But it could certainly head that way. As Senate Minority Leader Harry Reid puts it in a Washington Post op-ed, “If we enshrine this precedent and declare a functioning Supreme Court optional, subordinate to the whim of the Senate majority, it is easy to envision a future where the Supreme Court is routinely crippled.”

On the Trail

Hillary Clinton meets in New York with civil-rights leaders before delivering a speech on race at 3:15 pm ET… Bill Clinton stumps in South Carolina… As does Bernie Sanders, who hits Columbia and Charleston before heading to Georgia… Donald Trump holds rallies in North Augusta, SC and Beaufort, SC… Ted Cruz makes three stops in South Carolina… Marco Rubio and Jeb Bush also campaign in the Palmetto State.

Countdown to MSNBC Dem Townhall in Nevada: 2 days

Countdown to Dem Nevada caucuses: 4 days

Countdown to GOP South Carolina primary: 4 days

Countdown to GOP Nevada caucuses: 7 days

Countdown to Dem South Carolina primary: 11 days