While Democrats fret about how to win back control of the White House and Senate in 2020, they may have more luck rolling out a “national” governing agenda at the state and local level. New York choosing to enact congestion pricing on some Manhattan roads could provide the latest catalyst for a policy change in one Democratic city — creating momentum for changes in other Democratic-friendly jurisdictions across the country.

Even if a Democrat wins the White House in 2020, they might not be able to pass much at all in the way of policy. That’s because Senate math for Democrats is bleak, needing to win three net seats (plus the White House as a tie-breaker) just to win a majority in the chamber. Increasingly Democratic-friendly Colorado looks like the most likely pickup. Races in Maine and Arizona figure to be close, but both would be tossups for Democrats at best. After that would be seats in North Carolina, Georgia and Texas, where Democrats have been increasingly competitive at the presidential level, but all of those seats look like uphill battles. And then there’s the matter of Alabama, where Democrat Doug Jones won a special election in 2017 but goes into 2020 as an underdog.

Democrats winning the White House but not the Senate is a scenario they would rather not think about — but it is very plausible.

Which is why they should be thinking about a parallel “national local” agenda. Democrats being clustered in large metro areas may be geographically inefficient when it comes to congressional representation, but it’s handy when so many large cities in which Democrats live are controlled by Democrats. While there’s no official mechanism for cities to follow each other’s lead, that seems to be a growing pattern.

The most prominent example of this is probably the minimum wage. Earlier this decade the “Fight for $15” movement took hold, with activists seeking to raise the minimum wage to $15 an hour wherever they could get politicians to pass it. Seattle was one of the first cities to pass it, and other coastal cities like San Francisco and New York followed. A $15 minimum wage was added to the Democratic Party’s national platform in 2016. Even in cities and states that haven’t raised minimum wages to $15, some have increased theirs in part to respond to the momentum seen in high-profile Democratic cities.

One way of thinking about this dynamic is that cities like Philadelphia, Atlanta and Denver aren’t likely to pass certain types of legislation until they see it pass in places like New York, San Francisco and Los Angeles — and observe how it goes. It makes sense that the most progressive cities will grow fond of certain kinds of legislation — whether it be a $15 minimum wage or banning plastic bags — before more pragmatic cities do. So those progressive cities should go ahead and pass progressive legislation, giving other cities the real-world examples to decide it’s worth following suit.

That’s why congestion pricing coming to Manhattan is such a big deal. Even though other cities in the world have adopted it, somebody in the U.S. was going to have to go first, and it makes sense that it’s traffic-choked, transit-funding-starved Manhattan. Not only will other cities get to observe the impact congestion pricing will have on Manhattan traffic and transit revenue, but also politicians will get to observe how much public outcry there is once it’s in place. If things go smoothly, maybe it will spread to other congested cities in a few years.

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To the extent the Manhattan plan doesn’t go far enough — with “carve-outs” to allow many drivers into the crowded areas without paying — well, every new big idea has to start somewhere. It took a while for the income tax to come to the U.S., and its early editions had rates nowhere close to the levels of today. Congestion pricing can always be expanded with fewer exemptions in the future.

“Trickle-down progressivism” works like this. The biggest, most progressive cities, particularly in similarly Democratic states New York and California, elect the most progressive people possible. Then, those mayors and governors enact progressive legislation, which creates momentum for the next tier of cities and states who aren’t as bold as New York and California.

Democratic Americans in deep blue municipalities sometimes feel like their votes are wasted in presidential and congressional elections, but by electing local leaders who enact progressive policies and create momentum for similar laws elsewhere, they have the ability to be just as transformational.

Conor Sen is a Bloomberg Opinion columnist. He is a portfolio manager for New River Investments in Atlanta and has been a contributor to the Atlantic and Business Insider.