On ABC News’ “This Week” yesterday, George Stephanopoulos had to ask Scott Walker three times whether he favors changing the Fourteenth Amendment to end birthright citizenship before getting an answer. Three times. Here’s the final exchange:

STEPHANOPOULOS: So you’re not seeking to repeal or alter the Fourteenth Amendment.

WALKER: No. My point is any discussion that goes beyond securing the border and enforcing the laws are things that should be a red flag to voters out there, who for years have heard lip service from politicians and are understandably angry because those politicians haven’t been committed to following through on those promises.

As I noted last week, Walker’s ongoing answers to the birthright citizenship question actually reveal a deeper and more consequential series of evasions, on the core question of what we should do about the 11 million undocumented immigrants in this country. Walker’s answer yesterday was of a piece with that.

When the GOP candidates are pressed on what they would do about the 11 million, the results tend not to be pretty. For instance, on Meet the Press, Chuck Todd asked Carly Fiorina about Trump’s call for ending birthright citizenship –which Fiorina rejected far more forcefully than Walker did. But then Todd sensibly followed up with this:

TODD: What do you do with the 11 million?

FIORINA: My own view is, if you have come here illegally and stayed here illegally, you do not have an opportunity to earn a pathway to citizenship. To legal status, perhaps. But I think there must be consequence.

Fiorina says that “perhaps” undocumented immigrants should have a path to legal status — provided it precludes any chance at citizenship. Okay, if you’re not willing to support legal status, then what should be done instead? Walker, for his part, has declined to endorse mass deportations, but doesn’t think we should even talk about legalization until the border is secured.

If Trump’s GOP rivals are going to be pressed on whether they agree with his positions on immigration, the focus should be more on his vow to deport the 11 million than on his call for ending birthright citizenship. If they don’t support mass deportation, what do they support? And no, claiming you might support legal status once some undefined ideal of border security is attained isn’t a real answer.

Republican presidential candidate Donald Trump released a detailed immigration plan, and its "great, great wall" is just the beginning. (Gillian Brockell/The Washington Post)

The question of what to do about the 11 million is the fundamental underlying policy dilemma that is at the core of the whole immigration debate. And it’s one many Republicans have refused to reckon with seriously for years now. They’ve called for more “enforcement of the law” while taking care to avoid saying whether this means they want maximum deportations. And they’ve claimed to be open to legalization at some point later without meaningfully defining what conditions must be established first. This is roughly where Walker is now.

By putting the call for mass deportations out there that as an explicit policy goal, Trump has unmasked those evasions for what they are. Trump has provided an opportunity to pin down his rivals on the core immigration policy question we face. So one hopes we see more questioning along the lines Todd pursued.

Still, the birthright citizenship debate has been clarifying in one sense. There’s been a lot of talk about how Trump’s appeal is rooted more in attitude than in the specifics of what he’s saying. Yet some of the candidates appear to believe that the views of GOP primary voters require them to appear open to Trump’s call for ending birthright citizenship — which, like it or not, is a specific policy pronouncement — or at least to treat it gingerly.

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* HARRY REID ENDORSES IRAN DEAL: In an interview with Paul Kane, Dem Senate leader endorses the Iran nuclear agreement, giving it a big boost. And:

[Reid is] “cautiously optimistic” that he would be able to secure enough support to prevent an override of Obama’s veto of a resolution opposing the deal. An override would require opponents mustering 13 Democrats to join all 54 Republicans in opposition to the president. Reid said he was “still hopeful” that at least 41 Democrats would support Obama, which would suffocate the resolution in a filibuster and not require a presidential veto.

The latest whip count has 27 Senators backing the deal, meaning only seven more of the remaining 17 undecided Dems are needed to get the 34 necessary to sustain Obama’s veto. A total of 31 are either supporting the deal or leaning towards backing it.

* WHITE HOUSE MAKING NEW ARGUMENT FOR IRAN DEAL: Politico reports that administration officials are privately making the case that the Iran nuclear accord could actually make it easier to target Iran’s nuclear program with military action later, if necessary:

In meetings on Capitol Hill and with influential policy analysts, administration officials argue that inspections of Iran’s nuclear facilities under the deal will reveal important details that can be used for better targeting should the U.S. decide to attack Iran. “It’s certainly an argument I’ve heard made,” said Rep. Adam Schiff, the ranking Democrat on the House Intelligence Committee. “We’ll be better off with the agreement were we to need to use force.”

In other words, even if you think Iran will cheat, requiring a military response, why not have an inspection regime in place that will give us more information about its nuke program, rather than not have one in place? It’s an interesting species of bluff-calling.

* BIDEN ‘LEANING TOWARD’ RUNNING FOR PRESIDENT? So reports the Wall Street Journal:

Biden, who has long been considering a presidential bid, is increasingly leaning toward entering the race if it is still possible he can knit together a competitive campaign at this late date, people familiar with the matter said….conversations about the possibility were a prominent feature of an August stay in South Carolina and his home in Delaware last week, these people said.

Biden would start out far behind Hillary Clinton, both in polling and — crucially — in fundraising and organizing. His advisers say he’ll make a final decision by late September.

* GOP DIVIDED OVER OBAMACARE: The Hill has an interesting look at how the 2016 GOP candidates are divided over how far to go in proposing Obamacare repeal, and what that means politically:

The 2016 election will mark the first time Republicans will be running against Obamacare since its biggest pieces have gone into effect, including billions of dollars of subsidies that have helped millions to gain coverage. Already, GOP strategists are getting heartburn about how to fight against ObamaCare without turning away those who are benefiting.

Plans from Scott Walker and Marco Rubio would be significantly less generous than Obamacare is. But they are still a nonstarter for conservative rivals like Bobby Jindal because they seek to protect at least some of its beneficiaries by spending money to expand coverage.

* JEB BUSH HEADS FOR MEXICAN BORDER: Jeb Bush is set to make a stop today near the Mexican border, even as he continues to defend his use of the term “anchor babies.” It bears repeating: Bush’s call for enforcement against the supposed “anchor baby” phenomenon is now the position held by the “moderate” in the 2016 GOP field.

* AND TRUMP-MENTUM, EXPLAINED: The New York Times had an epic weekend piece of reporting into the sources of Donald Trump’s appeal, concluding from polls and interviews with voters that Trump-ism is rooted in attitude more than anything else:

His support is not tethered to a single issue or sentiment: immigration, economic anxiety or an anti-establishment mood….Trumpism, the data and interviews suggest, is an attitude, not an ideology….His most offensive utterances have, for many Republicans, confirmed his status as a unique outsider willing to challenge conventions, and satisfied a craving for plain-spoken directness.

And yet polls show that large chunks of Republican voters agree with Trump’s pronouncements on immigration, and his rivals think they need to gravitate towards his positions on the issue.