States in blue have expanded Medicaid.

President Barack Obama signs the Affordable Care Act. The law includes the largest expansion of Medicaid coverage for poor adults in the program’s history. The ACA creates a new minimum standard allowing legal U.S. residents with incomes just above the poverty level to enroll in the program. The federal government will cover no less than 90 percent of the new spending.

Five states and the District of Columbia begin phasing in the expansion early during 2010 and 2011.

The Supreme Court rules, 7-2, that states may opt out of the law’s Medicaid expansion without losing previous federal funding. The decision leaves millions of poor residents without health coverage in states that decide to reject broader Medicaid eligibility.

President Barack Obama wins re-election, enabling him to continue implementing the Affordable Care Act during his second term despite Republican calls for repeal.

Over the course of 2013, a number of states pass bills or take administrative steps to accept the ACA’s Medicaid expansion, which will take full effect on Jan. 1, 2014. Most of these states are run by Democrats, who adopt the policy with little fanfare. But a handful of Republican governors, starting with Brian Sandoval of Nevada, also announce their support.

Arkansas Gov. Mike Beebe (D) strikes a compromise with the majority-Republican legislature to pass the “private option,” a version of the Medicaid expansion that uses federal money to subsidize private insurance. This inspires the adoption of similar plans in states with Republican leaders and divided governments.

Arizona Gov. Jan Brewer (R) muscles a Medicaid expansion through the legislature, prompting a legal challenge from angered Republican legislators that a court later rejects.

Iowa and Michigan follow Arkansas’ lead when their Republican governors and legislatures enact privatized variations of the Medicaid expansion.

Ohio Gov. John Kasich (R) circumvents the Republican majority in the legislature and expands Medicaid via an obscure budgetary mechanism, a maneuver later affirmed by the state Supreme Court.

Medicaid coverage becomes available to newly eligible residents of states that joined the ACA’s expansion.

After a prolonged fight with the majority-GOP legislature, New Hampshire Gov. Maggie Hassan (D) enacts a privatized Medicaid expansion plan.

Gov. Tom Corbett (R) of Pennsylvania, a former Medicaid expansion opponent, signs a privatized version of the policy into law. Six months later, his Democratic successor, Tom Wolf, scraps it and begins moving enrollees into standard Medicaid.

The majority-Republican legislature and Montana Gov. Steve Bullock (D) agree on a privatized expansion of Medicaid. A year earlier, Medicaid expansion legislation had nearly passed, but a Democratic lawmaker accidentally voted against the bill, leaving it one vote short.

Indiana Gov. Mike Pence (R) and the Republican legislature enact the most conservative version of Medicaid expansion to date, which requires enrollees to shoulder more of their medical costs than under the traditional program.

Gov. Bill Walker (I) of Alaska ends months of fruitless negotiations with Republican legislators and expands Medicaid under his own authority, an action GOP lawmakers unsuccessfully challenge in court.

South Dakota Gov. Dennis Daugaard (R) reverses course and endorses the Medicaid expansion, but receives only tepid support from the legislature.

Newly elected Louisiana Gov. John Bel Edwards (D) enacts the Medicaid expansion shortly after taking office.

New Kentucky Gov. Matt Bevin (R) announces his plan for a conservative remake of his state’s Medicaid program, and threatens to rescind the expansion if the Obama administration doesn’t approve it.

Donald Trump is elected president on a platform that includes repealing the Affordable Care Act, a goal he shares with the Republican majority in Congress.

Following a post-election meeting with Vice President-elect Mike Pence, South Dakota Gov. Dennis Daugaard (R) drops his Medicaid expansion plan.

Republican governors including Charlie Baker of Massachusetts, Rick Snyder of Michigan, John Kasich of Ohio, Asa Hutchinson of Arkansas and Brian Sandoval of Nevada urge Congress to take care not to disrupt the Medicaid programs in their states by acting too swiftly to repeal the Affordable Care Act without a replacement in place.

The history of Medicaid suggests the expansion would continue to spread if the Affordable Care Act were left in place. When the program launched more than 50 years ago, just over half the states signed up in the first two years. The last state to join, Arizona, didn’t begin participating until 1982.

But repealing the Affordable Care Act would eliminate the federal funding that mostly finances the Medicaid expansion. States lack the resources to maintain this coverage without these dollars, meaning a large portion, at least, of those who’d enrolled under expansion would become uninsured.