WASHINGTON — Hillary Clinton assailed Donald J. Trump on Friday as untrustworthy on women’s issues, sharpening her tone against him in her first major speech since becoming the presumptive Democratic presidential nominee.

Finally free to focus on the general election, Mrs. Clinton signaled that she planned to hit her likely Republican opponent in an area of perceived weakness while working to mend fissures within her own party by focusing on issues dear to progressives, such as protecting abortion rights and funding for Planned Parenthood.

“When Donald Trump says, ‘Let’s make America great again,’ that is code for let’s take America backward,” Mrs. Clinton said at a Planned Parenthood Action Fund event in Washington.

In a 30-minute speech, Mrs. Clinton seized on Mr. Trump’s suggestion in March that abortion be banned and that women who violate such a ban face punishment. Although he later retracted the remark amid a bipartisan backlash, Mrs. Clinton made clear that she would not let it go.