As in 2016 and the 2018 midterms, health care has already emerged as a core issue, though there are fissures in both parties. Congressional Democrats have rallied around Obamacare, while some of the party's presidential nominees are supporting "Medicare for All" plans that would offer universal, government-backed health coverage.
President Donald Trump, who campaigned in 2016 on repealing the law, has promised that Republicans will pass a "really great" health care plan after the 2020 election, although none has been proposed.
The administration last month sided with Republican-led states that are pushing for the law to be invalidated by the courts.
