Mitt Romney’s not running for President again, telling supporters that it’s time for the Republican Party to look to other leaders for its future. Democrats may wonder whether they have to tell Hillary Clinton the same thing. In a PPP poll taken this week, Hillary Clinton can’t get to 50% against any Republican challenger paired against her, even Romney:

The automated poll of nearly 900 registered voters, conducted last week by Public Policy Polling, found that 48 percent of respondents had an unfavorable opinion of Clinton, compared to 43 percent who viewed the former secretary of State favorably.

While Clinton — the prospective favorite for the Democratic presidential nomination should she enter the race — holds leads over every major GOP candidate tested in the poll, she doesn’t break 50 percent against any, and some are well within striking distance. Wisconsin Gov. Scott Walker comes closest, with Clinton leading him by a margin of 45 percent to 42 percent (with 14 percent not sure who they’d vote for) – within the survey’s margin of error of plus or minus 3.3 percent.

It’s worth noting the provenance of this particular survey:

The poll was provided to POLITICO by one of the donors who funded it, who asked to remain anonymous. It does not directly ask respondents to rate Warren’s favorability or to choose between the Massachusetts Senator and Clinton, nor does it pit Warren against any of the prospective GOP candidates. But it appears to be part of a broader effort by liberal Democratic donors and activists to make the case that Warren, who has repeatedly insisted she has no interest in running for president, could defeat Clinton for the Democratic nomination and also would be a more viable general election candidate.

If that’s the case, then Warren supporters have lots of reasons to be disappointed in the results. In the demos, the sample splits 45/44 between Obama and Romney voters in 2012, so it’s relatively close to reality. Most of the questions that follow the candidate matchups look very much like a push poll, with the first one prominently mentioning Warren and her slogan that “the system is rigged” in favor of Wall Street and the big banks. The next eight questions alternate between painting Hillary as the tool of both, and Warren as the savior of the working class. (That’s only a slight exaggeration; read the questions for yourselves.) The next five questions attack Hillary on foreign policy, and then two more go after the idea of a Clinton dynasty and Hillary’s claims of executive experience.

It’s pretty much the Full Progressive Smash on Hillary, and her numbers are accordingly lopsided. She only gets 37/43 for the nomination by question 23, and then a 24/40 on whether her nomination would make voters more/less likely to vote for other Democrats in question 24. Even with all of that push polling and flattery, though, Warren only gets a 25/37 on the same coattail measure in question 25 – almost identical to Hillary. Warren supporters may have outdone themselves with this exercise.

This still doesn’t translate to good news for Hillary, though. Given her high name recognition and her four years in the Obama administration — plus the continuing popularity of Bill Clinton — she should be soaring over the little-known competitors in the GOP. Yet Scott Walker, who has barely engaged national media, comes within three points of Clinton in a D+2, 45/44 2012-Obama/Romney sample and holds her to 45%. That’s a bad sign for someone who’s more or less going to be in an incumbent-like position, and who has proven to be less than adept on a campaign trail in two different iterations — 2008 and her book tour last year.

This is the second poll this week that suggests that Hillary’s support is collapsing. Andrew Malcolm picked up a similar trend from a Zogby poll earlier in the week:

In just the past month, when no one with a real life was thinking about the 2016 presidential race, Hillary Clinton’s massive poll lead over every single potential Republican opponent has collapsed.

In a hypothetical match-up, her margin over Jeb Bush, the closest GOP president, was cut in half to eight points this month, 45%-37%, according to Zogby. That’s down from a whopping 15-point lead in December. Similarly, the former first lady’s 15-point margin over Mitt Romney slid to nine points, 46%-37%.

Still sizable margins, to be sure. But trending uncomfortably in the wrong direction even before an announcement, as Clinton goes to ground to plot her campaign, assemble staff and devise a credible rationale other than gender for why Americans should let her move back into the White House, this time in the West Wing.

Maybe the problem is that no one’s heard why Hillary wants to run for the office, other than to move back into the White House. According to one reporter, even David Axelrod isn’t clear “what are we ready for when people say Ready for Hillary” (via TWS):

http://www.youtube.com/watch?v=uhLD0BvF4s8

Chris Cillizza and Aaron Blake ask another question — if not Hillary, then who?

We are rapidly approaching the point of no return for Clinton. That is, if she were to suddenly take herself out of the race in, say, two months time, there would be a massive sense of doom within the party. The shock of the decision would reverberate for weeks — and maybe even months — making it hard for anyone looking to fill the void she left behind.

Now, that doom would eventually be followed by a wild scramble among the Bidens, Martin O’Malleys and, yes, even Elizabeth Warrens of the party for the donors, activists and staffers that had all been assumed to be part of the Clinton machine. But, doing things in a hurry with what would widely be regarded as Democrats “B” or even “C” team would be deeply problematic.

Simply put: For Clinton to pass on the race — and especially if she waits until summer to make her decision public — would be absolutely disastrous for her party’s chances of holding onto the White House next November. She and her budding team have to know that and it’s hard for me to imagine that she would have let things go this far — there is, literally, an entire campaign and outside group world already in place for her — if she had any serious or lingering doubts about whether she was going to make the race.

Democrats had better start looking for a Plan B, and it had better not be Elizabeth Warren, who can’t even win her own push polls.