The U.S. has reached the roughly half-year mark of a new phase in climate politics, which began when Democrats won House control and the party's 2020 primary campaign got rolling in earnest.

Why it matters: Democrats are beginning to shape the ideas that could become actual policy if the 2020 elections open a political window, while some Republicans are scrambling to come up with a response.

A non-comprehensive list of early political takeaways:

Ambition is in. At least 9 major candidates have endorsed moving the U.S. to net-zero greenhouse gas emissions by 2050, which is a very heavy lift. And we're seeing other aggressive ideas, including calls by Elizabeth Warren and other candidates to end new fossil fuel leases on federal lands.

At least 9 major candidates have endorsed moving the U.S. to net-zero greenhouse gas emissions by 2050, which is a very heavy lift. And we're seeing other aggressive ideas, including calls by Elizabeth Warren and other candidates to end new fossil fuel leases on federal lands. But so is realism . No major candidate (known to Axios) is calling for a 2030 net-zero U.S. emissions target, which has been the goal of some GND backers. Even the aggressive Sunrise Movement most recently said the U.S. has a responsibility to get there faster than 2050, but did not repeat their 2030 deadline.

. No major candidate (known to Axios) is calling for a 2030 net-zero U.S. emissions target, which has been the goal of some GND backers. Even the aggressive Sunrise Movement most recently said the U.S. has a responsibility to get there faster than 2050, but did not repeat their 2030 deadline. Climate has arrived in national politics . A recent CNN poll showed it among the very top priorities for Democratic primary voters, and it's not an isolated result. Presidential hopeful Beto O'Rourke made climate the subject of his first detailed policy proposal, and I'm confident that the days when it's absent from televised debates are gone.

. A recent CNN poll showed it among the very top priorities for Democratic primary voters, and it's not an isolated result. Presidential hopeful Beto O'Rourke made climate the subject of his first detailed policy proposal, and I'm confident that the days when it's absent from televised debates are gone. It's a mixed bag for carbon pricing . An ostensibly bipartisan House carbon fee bill has just 1 GOP backer so far. Nor is pricing — via a tax or permit trading — a priority for key GND advocates, though they do see some role. Via NYT, several major candidates support pricing as part of wider climate policies. But it has not been the central pillar of discussion on the stump thus far.

. An ostensibly bipartisan House carbon fee bill has just 1 GOP backer so far. Nor is pricing — via a tax or permit trading — a priority for key GND advocates, though they do see some role. Via NYT, several major candidates support pricing as part of wider climate policies. But it has not been the central pillar of discussion on the stump thus far. GND also has mixed bag . AOC and activists — including Sunrise — have pushed the GND to the forefront of the conversation. But backers have made some mistakes, and the Senate version of the resolution has gained just 1 new co-sponsor since its February rollout with 11 supporters (including many 2020 candidates). And the national AFL-CIO has been critical, which matters as Joe Biden — who's atop the early polls — seeks to run as a labor candidate.

. AOC and activists — including Sunrise — have pushed the GND to the forefront of the conversation. But backers have made some mistakes, and the Senate version of the resolution has gained just 1 new co-sponsor since its February rollout with 11 supporters (including many 2020 candidates). And the national AFL-CIO has been critical, which matters as Joe Biden — who's atop the early polls — seeks to run as a labor candidate. Republicans need … something. A number have floated ideas of their own or are working on them. Sen. John Cornyn, who's in Republican leadership, tells Bloomberg he's working on an innovation-focused proposal, while Axios' Amy Harder looks at other Republicans here. GOP plans are far less aggressive than Democratic ideas, but the politics have moved away from simple opposition.

Editor's note: This story was updated to clarify that multiple Democratic candidates support an end to new federal fossil fuel leases.