Democrats quickly accused the governor of trying to bolster his standing with Maryland voters in an election year.
"This is a political calculation, pure and simple," said Sen. Richard S. Madaleno Jr. (D-Montgomery, who is seeking the Democratic nomination for governor. "If it was genuine, he could have jumped on board earlier like several other Republican governors."
Hogan spokesman Doug Mayer said Wednesday that the governor decided to join after confirming that the alliance was taking real action to help states address climate change.
"We wanted to make sure this was something that was substantive and nonpartisan," he said. "Far too often in today's discussions about the environment, it's mainly focused on attacking people instead of working together."
Fourteen other states, Puerto Rico and the District have joined the U.S. Climate Alliance, committing to uphold the emissions-reduction targets of the Paris climate accord, which the United States signed on to under former president Barack Obama.
After Trump withdrew from the Paris accord, Hogan said he disagreed with the decision. But he balked at calls to join the U.S. Climate Alliance, questioning the intent of the group and the need for Maryland to be involved with it when the state's carbon-reduction goals were already stronger than those in the international agreement.
Whereas the Paris agreement calls for the United States to reduce its emissions by about 28 percent by 2025, Maryland's Greenhouse Gas Reduction Act, which Hogan signed two years ago, requires the state to cut its emissions by 40 percent by 2030, using 2006 levels as a baseline.
In his letter, Hogan touted Maryland's efforts to reduce carbon emissions during his tenure in office, saying he and other state officials "will share our insights, experiences, and strategies in meeting and excelling beyond requirements of the Paris climate accord."