Be careful what you wish for, Republicans. (Melina Mara/The Washington Post)

THE MORNING PLUM:

Senate Republicans appear to be, if anything, closing ranks behind Mitch McConnell’s vow that the Senate will block anyone nominated by President Obama to replace Antonin Scalia on the Supreme Court. Even vulnerable Senate Republican incumbents are now rallying behind McConnell’s hard line.

But in so doing, Republicans may be courting a pretty serious risk: if Obama doesn’t end up getting his pick through, there is a real possibility that President Hillary Clinton could nominate a more liberal replacement than Obama did — and she might also enjoy a Dem Senate majority, making it easier to get that more liberal replacement confirmed.

Most analysts think that the outcome of the presidential race will play a substantial — though perhaps not decisive — role in determining the outcome of the most contested Senate races, many of which are taking place in key presidential swing states. And so, if Clinton does get nominated and then win the White House, that perhaps boosts the chances of Dems taking back the Senate, too.

Former top aides to Obama believe he is likely to nominate a potential Scalia replacement who might have at least a marginal chance of getting some GOP support. (One potential nominee, Sri Srinivasan, is “considered a moderate,” has been praised by Republicans, and was confirmed to the D.C. Circuit by a 97-0 vote.) Obama might have a strong interest in picking someone with a chance of getting confirmed, since it would allow him to further shape the High Court before leaving office.

On the other hand, President Hillary Clinton might end up being less constrained in terms of her pick — in 2017, Dems may control the Senate, or barring that, the GOP majority will likely be a lot slimmer. “If Republicans refuse to consider a nominee, they’re likely giving the next president much more leeway in selecting a justice and controlling the direction of the court,” Jonathan Adler, a law professor at Case Western Reserve University, tells me. “That could mean a Republican president selecting a suitable heir to Scalia. But it could also mean a Democratic president nominating an anti-Scalia.”

Now, obviously, this may be a gamble — Adler calls it a “principled risk” and an “all-in bet on the future of the Court” — that Republicans are willing to take. Broadly speaking, the promise of keeping alive the possibility of a GOP president getting to replace Scalia with another conservative might weigh more heavily than the risk of getting a more liberal justice than Obama might nominate, since the court would be tilted in a more liberal direction under either Democrat’s nominee. Also, Republicans could presumably filibuster a Clinton nominee if they do lose the Senate, though they would be in a considerably weakened state after such a big loss, perhaps making that harder to sustain.

McConnell may also be calculating that if Republicans do lose the White House, they could then quickly confirm Obama’s nominee. “If Obama makes a relatively moderate choice, and Republicans lose the election, don’t be surprised if there’s a lame duck confirmation, so Hillary doesn’t pick someone further to the left,” Adler notes.

Of course, even if Republicans may have good reasons for refusing to consider and vote on Obama’s nominee, they may also have no choice. Even vulnerable Senate GOP incumbents such as Rob Portman of Ohio, Kelly Ayotte of New Hampshire, and Ron Johnson of Wisconsin have rallied behind this new cause, even though, as Nate Cohn points out, opposing Obama’s pick could alienate the moderate voters in these swing states that might help determine their reelection chances. Yet they are digging in, surely in part because conservative groups are demanding that Republicans show maximum resistance to any Obama nominee.

So this could end up unfolding as another example of the GOP base’s demands ultimately hurting the party over the long term.

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* TRUMP STILL DOMINATES NATIONALLY: The latest NBC/Survey Monkey poll finds that Donald Trump still holds a commanding lead among registered Republicans nationally: He has 38 percent, to 18 percent for Ted Cruz, and 14 percent for Marco Rubio. All the rest are in single digits.

One big question is whether Jeb can post a very good showing in South Carolina to rescue his candidacy; in the new poll he’s at all of four percent nationally. Also: the percentage of Republicans who think Trump will win the nomination is up to 56 percent.

* TRUMP, CLINTON DOMINATE SOUTH CAROLINA: A new Public Policy Polling survey finds Donald Trump with 35 percent in South Carolina, while Rubio and Cruz are tied for second place at 18 percent apiece. Meanwhile, Hillary Clinton leads Bernie Sanders in the state by 55-34, perhaps illustrating the demographic challenges Sanders will now face.

If Rubio comes in second in South Carolina, the roar of “Marco-Mentum” and “Rubio rebounds” punditry will be deafening.

* HARRY REID TURNS UP HEAT ON REPUBLICANS: Dem Senate leader Harry Reid has a new piece in The Post blasting Republicans for signaling a refusal to vote on an Obama nominee to replace Scalia, arguing that it could render the Senate (even more) dysfunctional for years:

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.If my Republican colleagues proceed down this reckless path, they should know that this act alone will define their time in the majority….they will be unconditionally surrendering their party to hard-line presidential candidates Sen. Ted Cruz and Donald Trump.

Of course, this is a political fight, and the goal (in addition to prodding Republicans to consider a Scalia replacement) is to extract as much political pain as possible as long as they fail to do so.

* SUPREME COURT JUSTICES HAVE BEEN CONFIRMED IN ELECTION YEARS: The New York Times has a useful chart that lays out this conclusion:

Since 1900, the Senate has voted on eight Supreme Court nominees during an election year. Six were confirmed. But several of those were for seats that had become vacant in the previous year. The Senate has never taken more than 125 days to vote on a successor from the time of nomination; on average, a nominee has been confirmed, rejected or withdrawn within 25 days.

If it is true that the Senate has never taken more than 125 days to vote on a nominee, it’s worth noting that there are nearly double that number of days left in Obama’s second term.

* HILLARY TO SPEAK ABOUT ‘SYSTEMIC RACISM’: Hillary Clinton is set to deliver a speech today that is focused entirely on “systemic racism”:

Her approach appears designed to highlight the contrast with her chief rival Bernie Sanders, who so far in his campaign has mainly addressed racism through the lens of economic inequality….Clinton is expected to call for “new investments in job creation to ending redlining to ensuring equal pay for women of color to ending the school-to-prison pipeline,” according to a campaign aide.

Clinton is arguing to the more diverse electorates in coming Dem primaries that Sanders’s inequality focus is too narrow to address the problems that various Dem constituencies face.

* AND REPUBLICANS PREP FOR BROKERED CONVENTION: Politico reports that GOP presidential campaigns are privately preparing for a possible brokered convention, should Donald Trump or Ted Cruz emerge at the top. Note this, from former RNC chair Michael Steele:

“If they want to monkey around with this process and try to fix it, they’re asking for all hell to break lose,” he said. “Any inkling that state party officials or national party officials are colluding and conspiring to prevent a particular individual from getting the nomination,” he said, “will basically create Armageddon with the base.”

On the other hand, a Trump or Cruz nomination could prove even more disastrous for the party!