WASHINGTON (AP) — Liberal interest groups are intensifying pressure on Democrats to take aggressive measures to reshape the Supreme Court, arguing the party should include such measures in its policy platform at next month's convention.

Twenty-two Democratic-aligned groups signed a letter sent this week to the party's platform committee, saying Democratic electoral victories by themselves won't be enough.

The activists have already faced disappointment this year. Presidential candidates who embraced sweeping court reforms, such as former South Bend, Indiana, Mayor Pete Buttigieg dropped their campaigns. The presumptive nominee, Joe Biden, has not gone as far, but he has pledged to nominate a Black woman to the high court.

But the liberal groups are pointing to a string of recent victories by progressive candidates at the congressional level as fresh proof that the party base wants the court to change to weaken the power of conservatives.

"Failing to include any provision on Court reform in the Party platform would send Republicans the message that they can continue to break the rules to hijack our courts because Democrats will do nothing to reverse their illegitimate gains," the letter said.

The effort is picking up steam even as liberals won some surprising victories, including on immigration, abortion and LGBTQ rights, from a court with five Republican-appointed, conservative justices.

In New York's Democratic congressional primary last month, Mondaire Jones and Jamaal Bowman, who both favor court expansion, won their party's nominations for seats in and near New York City. "Our democracy is under assault, and the Supreme Court has dealt many of the sharpest blows. If Democrats want to do something about that, expanding the Court is our only option," Jones wrote on Facebook in April.

While they won't have a say on the court, their victories are helping build interest in an area in which Republicans have long found it easier to motivate their most ardent supporters, said Brian Fallon, executive director of Demand Justice, the group leading the call for court reform in the Democratic platform. "There are some signs this election cycle that there is heightened awareness on the left of the threat posed by the sheer number of conservative nominees Trump has installed," Fallon said.

Biden has made court appointments a leading issue in his campaign with his pledge to put the first Black woman on the court and has suggested vetting is already underway to consider candidates if he's elected.

President Donald Trump similarly energized conservatives in 2016 by releasing a list of potential nominees to the high court. Trump has appointed 200 federal judges, including Justice Neil Gorsuch and Justice Brett Kavanaugh. The task has been made easier by changes in Senate rules, first adopted when Democrats held a majority, that eliminated the procedural rule that required 60 votes to confirm executive branch and judicial nominees.

The liberal groups say doing nothing is not an option, complaining Republicans have made "illegitimate gains" in the federal judiciary, embodied most vividly by GOP resistance to filling a Supreme Court seat in 2016 when President Barack Obama was in office, then quickly confirming Gorsuch after Trump won the presidency.

The court's size can be changed by legislation. The number of high court seats varied during its first 80 years from a low of six at the time the Constitution took effect in 1789 to a high of 10 during the Civil War. The current tally of nine justices was set in an 1869 law.

Congress might also act to impose term limits, but any change is likely to draw a legal challenge because the only limit set by the Constitution is that federal judges "shall hold their Offices during good Behaviour." They can be impeached, but otherwise decide for themselves when to hang up their robes.