Dear Democrats: It's too late to start over.

As in, there's no replacing Hillary Rodham Clinton as your party's front-runner for the presidential nomination. Not with Vice President Joe Biden — even if he runs. Not with former Vice President Al Gore. (I mean, come on.) Not with your ideal rich-person-with-no-record-and-a-fresh-faced-appeal.

This may come as a shock to some of you. After all, you argue, it's only August! The Iowa caucuses won't be until February! That's, like, four political lifetimes.

Sure. But modern presidential politics isn't as simple as announcing that you've decided to run and watching yourself soar to the top of the polls. (Unless, of course, you are Donald Trump, who appears entirely immune to every political law of gravity.)

Let's take Biden, for example. If he decides at the end of this month that he wants to run, he immediately begins in a $45 million (and probably much larger) hole against Clinton.

Super PACs could make up some of that ground, but remember that virtually every major fundraiser in the party — including many who were once Biden people — is now on Clinton's team. So whatever Biden raises, Clinton almost certainly raises double. Maybe triple.

Organizationally, Biden would start even further behind. Via Ready for Hillary, a super PAC dedicated to preparing the way for her presidential bid, Clinton's forces have been organizing in early states for well more than a year. And it's not just the head start; it's that Clinton had the pick of the staff in every single state. Outside of a few Biden loyalists who have been waiting for him to make up his mind, the talent pool has been drained.

The simple fact is that getting into a presidential race this late — and, yes, August before the election year is very late — has a disastrous recent history. Then-Texas Gov. Rick Perry (R) entered the 2012 race in August 2011 and within three months had become irrelevant. Former Sen. Fred Thompson of Tennessee got into the 2008 Republican race late — and never went anywhere. Ditto Wes Clark in the 2004 Democratic nomination contest.

What all of those failed candidacies have in common is that their best day — at least in terms of polling — was their first day. It was all downhill from there.

Why? Because people like a new thing, particularly if they have been staring at the same old things for a while and are not totally sold on any of them. Sure, Biden might get an initial boost in the polls. But the structural problems inherent in his candidacy would probably assert themselves quite quickly.

If Democrats wanted a serious primary fight between Clinton and someone else — it's hard to imagine who — that decision needed to have happened a year ago.

At that time, there was a lining up behind Clinton from the donor and establishment classes — a sort of mutually agreed upon decision that it was her time and that, despite the problems she might have, she represented the party's best chance to hold the White House in 2016.

All of the Democratic eggs were put in Clinton's basket, a remarkable and rare decision for someone who was neither a sitting president nor vice president. (Somewhere, Biden is nodding and muttering under his breath.)

Every leading consultant, fundraiser, donor and staffer sought to get on board either her campaign or the super PAC dedicated to electing her. Amid that blur of bandwagon-jumping, the very real concerns that people (inside and outside the party) had about Clinton got lost — or ignored.

But ignoring those doubts and worries didn't make them disappear. And so Democrats now find themselves — less than five months after Clinton declared her candidacy — faced with this stark reality: They have helped make Clinton, who is showing major weaknesses on honesty and trust issues and who could be a problematic general election candidate, very, very hard to beat in a Democratic primary.

The race is the race. It's Clinton as favorite — wounded but not mortally — with Sen. Bernie Sanders of Vermont as the liberal alternative and former Maryland Gov. Martin O'Malley leading a pack of also-rans and never-will-bes. If Biden runs, he could complicate Clinton's unified establishment support but would be very unlikely to beat her.

The idea of a late entrant reshaping the race — and unseating Clinton — might make sense to doubting Democrats. But the nature of the modern presidential contest makes it more fantasy than reality. Democrats threw their lots in with Clinton more than a year ago. Now they just have to try to ride it out. They don't have any choice. Not really.