American politics, for better or worse, is built on a two-party system that is occasionally challenged by independent third parties, who never have much in the way of notable successes. How many members of Congress are there from the Green Party? How many did H. Ross Perot get elected when he launched the most successful third-party bid for the White House in a generation? The answer to both is, of course, "zero." Third parties can change the political conversation in major ways (and occasionally even move the "Overton Window" in a big way), but so far none of them has built up enough success to truly challenge the dominance of the Republican/Democratic dichotomy. Instead, what is much more common is one (or both) of the two major parties being dramatically changed from within.

Most recently, we have seen this happen in dramatic fashion with the Republicans. Donald Trump co-opted the party apparatus and remade it into being his very own brand. It is, for all intents and purposes, now "The Trump Party," as it has abandoned much of what Republicans traditionally stood for (treating Russia as an adversary and championing free trade both immediately spring to mind, although there are plenty of other examples as well), in exchange for supporting whatever Trump supports without regard to any traditional conservative ideological basis. About the only thing Trump fully agreed upon with the traditional GOP has been tax cuts for wealthy people -- anything and everything else in the conservative ideological mix has been fair game for Trump to just toss overboard.

Trump's takeover of the Republican Party is notable in many respects, but the most startling thing about it was how quickly it happened. During his first term as president, there were indeed members of his own party who pushed back against his impulses, but they are now largely gone -- either retired from politics or primaried by pro-Trump candidates or just cowed into obedience by the threat of Trump's wrath. There simply is no meaningful pushback from the GOP to Trump anymore in his second term.

Currently the Democratic Party seems ripe for reinvention as well. The party's leadership is seen by the voters as far too timid and ineffectual in fighting back against Trump. The only visible energy in the party seems to be coming from those who want to seriously re-engineer what the party stands for. Many have compared this to the original shakeup which happened within the Republican Party, by calling it "the Tea Party of the left." The Tea Party was the first major indication that the GOP could be transformed, although there's no real direct connection between them and Trump (Trump did not get his start as a Tea Partier and then rise within the Republican ranks, he just saw the opportunity to rebrand the party from the outside and took advantage of it).

The Democratic Party could go through a transformation if a charismatic leader appeared and built a movement among the voters that was strong enough to overwhelm the ineffectual party leadership. It would be possible for a Trump-like figure to do so, since the party apparatus is so weakened and so uncertain of which direction it should head right now.

Several people seem to be attempting to do so to some degree or another, although success certainly isn't guaranteed for any of them. What they all have in common seems to be a fighting spirit and lots of energy -- which is precisely what the rank-and-file voters are currently longing for.

Three stories appeared in the news today that got me thinking about all of this. The first was a report of how Bernie Sanders and Alexandria Ocasio-Cortez are drawing what can only be called massive crowds on their "Fight Oligarchy!" tour:

Roughly 36,000 people in Los Angeles. More than 34,000 attendees in Denver. And another 30,000 on Tuesday night near Sacramento.

Those monster crowds -- more than 200,000 people in all, according to organizers -- have turned out to cheer on a fiery anti-Trump, anti-billionaire message from Senator Bernie Sanders of Vermont and Representative Alexandria Ocasio-Cortez of New York during their "Fighting Oligarchy" tour of Western states.

. . .

The day before their Folsom rally, Mr. Sanders and Ms. Ocasio-Cortez drew 12,500 people at a stop in Nampa, Idaho, according to a Sanders spokeswoman, who said it was the largest political event in the deep-red state since Barack Obama visited in 2008. His staff said the crowd of 36,000 in Los Angeles last week was the biggest of Mr. Sanders's career.

The enormous turnout has surprised even Mr. Sanders's staff members, who have had to switch to larger venues to accommodate the crowds. In Folsom, attendees waited in a line three miles long to get in, the Sanders spokeswoman said, with thousands peering through fences and watching from nearby hills.

Sanders and Ocasio-Cortez have a solid ideological platform -- one of true economic populism. But, as one Democratic strategist quoted in the article pointed out, it's more about the energy than anything else right now: "This is not about left versus center -- this is about, are you willing to fight or not? That's it."

However, neither Sanders nor A.O.C. are likely to charismatically remake the Democratic Party and lead them back into the Oval Office on their own any time soon. Sanders is almost certainly not going to run for president in 2028 (he's already 83 years old), and A.O.C. is so young (35) that she just recently qualified as being old enough to run for president herself -- and it is much more likely that she will challenge Senate Minority Leader Chuck Schumer in a primary next time around. But the movement to return the Democratic Party to its working-class roots could indeed have at least some success, especially with the dearth of energy from the party leaders and party establishment in the face of Trump right now.

The second article reported on the efforts of a House Democrat who also appeared onstage at the Los Angeles "Fight Oligarchy!" rally: Ro Khanna. The article's title pretty much says it all: "Ro Khanna Is Everywhere -- And He Would Like Your Attention." And while it is doubtful whether either Sanders or A.O.C. will run for president next time around, it's a pretty safe bet that Khanna will.

Khanna's ideology is more mixed, as he is somewhat of a technocrat at heart (which makes sense, since he represents a Silicon Valley district). He has plenty of policy ideas, which he outlined in a rather scattershot way in a New York Times article a few months ago. His ideas might be summed up as Democrats coming up with a solid economic industrial policy (something Joe Biden made inroads on, actually).

Khanna, like Bernie and A.O.C., know full well that what voters are hungry for right now is some energy. And like Pete Buttigieg, Khanna is not afraid to appear on right-wing media outlets in an attempt to get his message across. He's held town hall meetings in California's Central Valley, in an attempt to flip some House districts blue. His unsubtle message: it is time for "the old guard to go." And, in a more positive vein, "My view is, after eight years of Trump's sloganeering, people are actually going to want true solutions."

Many Democrats currently see Khanna as a kind of flash in the pan -- someone who is interesting to listen to, but also someone who likely would never convince enough primary voters to win the party's presidential nomination. This impression could change, over time, however.

The third article that caught my eye today is about someone who is far too young to even contemplate running for president any time soon. David Hogg is a survivor of the Parkland, Florida school shooting who went on to become a gun-control activist (he co-founded March For Our Lives). Hogg is now the youngest-ever (only 25 years old) vice-chair of the Democratic National Committee. His new group, "Leaders We Deserve" (which he co-founded with "the campaign manager for the first Gen Z member of Congress") is going to target sitting Democratic House members they see as insufficiently effective (or, as the other co-founder puts it, "asleep at the wheel" or "out of touch"):

"The reason we're doing this is to get the Democratic Party into fighting shape," Hogg said in an interview. "Right now it has become abundantly clear that the base of our party feels like we are not doing anywhere near enough... to stand up to Donald Trump."

Their efforts will be limited to targeting only Democrats in safe, deep-blue districts. They insist they will not back primary challengers in swing districts, where such efforts could backfire in the general election and hand seats to the Republicans. They also insist that they're not just doing this on the basis of age, although the group will also "continue its efforts to elect young candidates -- under 35 in federal races and under 30 at the state level -- in open and GOP-held seats."

There doesn't seem to be a strong ideological basis for their efforts, they just want to elect Democrats who are willing and able to fight back. It's about the energy level, not about the policy, to put it another way.

It is too early to say whether any of this will be successful in any meaningful way. Will any of it truly change the Democratic Party from within? None of it is truly a "Tea Party of the left," or at least not yet. The thread that connects these efforts is their quest to inject some new fighting spirit into a party that is seen as being more moribund and sclerotic than energy-infused.

Seeing such signs of energy, however, is certainly inspiring. There is obviously an audience out there hungry for such signs from Democrats. Bernie Sanders never drew a crowd of 36,000 people in either of his presidential bids, but now he is filling venues to capacity everywhere he goes. Whether the rest of the party establishment realizes and capitalizes on this hunger remains to be seen, at this point. But it does seem to be a "lead, follow, or get out of the way" moment for Democrats, and it is heartening to see at least a few people out there who are rising to the occasion.

-- Chris Weigant

Follow Chris on Twitter: @ChrisWeigant