Call them the sleeping giants of the 2018 midterms.

While much of Washington is transfixed with which party will control the House and Senate after November, there is a good argument that the fate of much of President Trump’s agenda will rest on the fate of a key handful of races for state attorneys general.

Attorneys general at the state level, often acting in partisan packs, have become a legal force multiplier in recent years. During the Obama administration, Republican attorneys general such as Scott Pruitt of Oklahoma (until last week President Trump’s administrator of the Environmental Protection Agency), Greg Abbott of Texas (now the Lone Star State’s governor) and Patrick Morrisey of West Virginia (now running for U.S. Senate) repeatedly led the charge in court against the Democratic administration’s regulatory agenda, social policies and Obamacare.

Mr. Abbott described his job in a 2013 Associated Press interview as this: “I go into the office, I sue the federal government and I go home.”

Under President Trump, it has been Democratic attorneys general such as Xavier Becerra of California and Bob Ferguson of Washington state who have taken on the Republican administration over the travel ban, climate regulations and, just last month, the policy of separating families at the border with Mexico.

“The Trump administration doesn’t understand the rule of law,” Sean Rankin, executive director of the Democratic Attorneys General Association, said in an interview. That, he added, is why “Democratic attorneys general are winning in court.”

But Zack Roday, communications director for the Republican Attorneys General Association, said it was important to increase the Republican ranks of attorneys general because Democrats tend to use the post for activism and indiscriminate opposition to President Trump’s policies.

Republican attorneys general, he said, “are dedicated to the rule of law and keeping their communities safe.”

Throw in the fact that the state’s attorney general post has typically been a key staging ground for higher office, and this year’s races will likely have an even larger long-term impact.

Some 30 states and the District of Columbia have attorney general races this fall, and Democrats are hoping for a number of pickups where Republican incumbents are not seeking re-election. Republicans are defending 18 positions, and seven of those races are considered toss-ups, according to a survey by veteran analyst Louis Jacobson in Governing magazine. Just three Democratic attorneys general are believed to be in competitive races.

Republicans control 27 attorneys general offices, compared with 22 for Democrats and one nonpartisan seat appointed by independent Alaska Gov. Bill Walker.

Even with the power of state attorneys general, the races often don’t generate a lot of voter excitement, said Mr. Roday.

“We have to fight for oxygen,” he said, adding that voters need to understand that the attorney general “is their fighter.”

On the offensive

Tim Jost, a former law professor at the College of William & Mary, said Republican attorneys general who fought rearguard action against the implementation of Obamacare can now go on the offensive with a friendly administration in Washington.

In February, a group of Republican attorneys general filed a lawsuit against the government, arguing that since Mr. Trump succeeded in repealing the Obamacare “tax” underlying the mandate to purchase health insurance, the state-based health care markets in the Affordable Care Act were unconstitutional. The Trump administration responded by telling a federal court that it does not plan to defend President Obama’s health care reform in the lawsuit.

Mr. Jost said the Obamacare fight dramatically illustrated the stakes involved in the attorney general races this fall.

The Trump administration has “made a political decision to throw a hand grenade into the ACA market. … The [Republican] attorney generals are dropping an atomic bomb on the ACA.”

Since 2016, Mr. Jost said, “Democrats are now following the same playbook,” filing lawsuits against the Trump administration on issues such as immigration. Last week, 18 states filed a lawsuit against the Trump administration to end family separations at the border.

Mr. Jost pointed to Virginia as an example of how attorney general races can impact elections.

Republican Kenneth T. Cuccinelli II gained national attention as the state’s first attorney general to challenge Obamacare in court. In his run for governor in 2013 against Democrat Terry McAuliffe, Mr. Cuccinelli told The Atlantic magazine, “We need people to know that November 5 is a referendum in Virginia on Obamacare.” Mr. Cuccinelli lost that election by about 40,000 votes.

Beyond the legal issues, the outcome of November’s races may have an impact on Mr. Trump personally.

New York Attorney General Barbara Underwood, a Democrat picking up where Eric Schneiderman left off when he resigned amid a sexual harassment scandal, has sued the president and Ivanka, Eric and Donald Trump Jr., claiming Mr. Trump’s charitable foundation was used improperly to benefit the Trump family.

It was Republican attorneys general who forced the issue on a key Supreme Court abortion case, filing a brief in the high court ruling that the state of California can’t force pro-life pregnancy centers to advertise abortion because the regulation violated the center’s First Amendment rights.

James E. Tierney, the Democratic attorney general of Maine in the 1980s and a Harvard Law School lecturer, said that suing a presidential administration is not a huge part of an attorney general’s role, but the lawsuits often attract outsized national attention.

Mr. Roday said he feels confident in the candidates this year.

“We do have prototypical candidates in every race,” experienced prosecutors prepared for the job, he said.

Lizzie Ulmer, communications director for the Democratic Attorneys General Association, said the group hopes to make inroads this cycle in the Deep South, including an open seat in Florida where Republican Attorney General Pam Bondi is prohibited from seeking another term.

This year’s election cycle presents “a big map, with lots of opportunities, and we are looking forward to November,” said Ms. Ulmer.