Just how far is Hillary Clinton in the lead for the 2016 Democratic presidential nomination?

The short answer is: by a lot, but it's hard to be any more precise.

Remember, we're in the “invisible primary” stage, in which party actors — politicians, party-aligned groups, campaign and governing professionals, activists, formal party officials and staff — compete and co-ordinate over candidates. We're still two years from the voters of Iowa and New Hampshire getting involved, but for people whose business or passion (or both) is party politics, the nomination contest has been under way for at least a year, and probably a lot more.

It's hard, however, to measure what's happening (hey, it's not for nothing that would-be observers wound up calling this phase “invisible”). The evidence suggests that party actors communicate with each other to some extent through high-profile endorsements. Fundraising matters too, of course, especially to those within the party network. So does recruitment of staff. In each of those realms, the universe of party actors is large, but signs that one candidate is winning a lot of support — along with a large share of party-controlled (or at least party- connected) resources — is a good sign that a candidate is performing well in the invisible primary.

Evaluating that sort of evidence isn't an exact science, but it's probably better than just listening to Washington conventional wisdom, which often mistakes success among one group of party actors for a national consensus (or, worse, assumes that national polling is a good proxy for support within the parties).

In recent weeks, evidence of Clinton's dominance has started to add up.

— Former Obama operative Jim Messina and the Priorities USA Action super PAC declared for Hillary. Another Super PAC, Ready for Hillary, is receiving support from some party heavy hitters.

— A survey of Congressional Democrats noted 59 Members of Congress ready to endorse Clinton once her formal campaign is launched; that's over one fifth of all House and Senate Democrats.

— Clinton appears to have a broad coalition of party actors beginning to organize in Iowa.

None of this precludes another candidate seriously competing against Clinton. For example, the congressional endorsements still leave the majority of elected Democrats uncommitted.

And some of her momentum results from simple bandwagon effects; as long as it appears that Clinton may sweep to an easy victory, few political players want to be on her bad side. We don't know how many of her supporters are truly dedicated to her campaign, and how many are just keeping their options open. If a serious challenge emerges, some weak supporters may switch.

Then again, at some point it doesn't matter. The reason frontrunners often win is precisely due to the power of the bandwagon, not because they have enthusiastic support from more than half of the party.

On the other hand, Clinton is almost certainly not there yet. Her 2016 campaign is still untested. At some point in 2014 or 2015, Clinton will have a bad week, and probably a bad month. She'll say something unfortunate (or perhaps her husband will), or someone will dredge up an old story — or even a new story — that makes her look bad. Or perhaps some high-profile party actor will come out against her. What happens then? We don't know, and can't know, whether her bandwagon supporters get antsy and begin looking for alternatives or hunker down to defend her claim to the nomination. (Of course, there's always the possibility that she'll drop out of the race before it formally begins.) We do know that when it happens, the news media will be eager to pile on, if for no reason other than their own strong interest in a competitive nomination fight.

Can she really sweep the nomination without serious opposition? Sure. Why not? It's never happened — aside from incumbents running for re-nomination — in the modern era, but the Gore/Bradley contest in 2000 wasn't that far off. As the parties have become better at coordinating and selecting nominees, longshot candidates have more reason to decide that contesting a nomination against a dominant frontrunner just isn't worth it. It's not 1976; no one is going to win by accident.

It's also very possible that one or more candidates currently (albeit fairly quietly) running for the 2016 Democratic nomination will step up their efforts. Again: we can assess nomination odds and the incentives for running or passing, but we can't really get inside a candidate's head and know how much he or she wants the nomination, or how deterred he or she might be by disincentives.

Which, alas, doesn't leave us with a clear conclusion. Clinton is certainly a strong frontrunner. Is she as strong as Mondale in 1984, or Gore in 2000? Stronger? All we can say for now is that she's in excellent shape. We'll just have to wait to see how excellent.

Jonathan Bernstein covers U.S. politics for Bloomberg View. He is co- editor of “The Making of the Presidential Candidates 2012.” Follow him on Twitter at @JBPlainblog.