CINCINNATI — In December, 13 of the 14 Democratic women serving in the U.S. Senate hosted a fundraiser for their favorite primary candidate: Hillary Clinton. The one who didn’t show? Massachusetts Sen. Elizabeth Warren.

Six months later, there’s little distance between two of the most powerful women in the Democratic Party.

With the primary season over and Vermont Sen. Bernie Sanders fading from the spotlight, Warren is stepping up to reclaim her role as leader of the party’s progressives. She’s mobilizing behind Clinton, lending her presidential bid a powerful boost of liberal credibility.

In recent weeks, the two women have formed a tight electoral alliance — one that could grow even closer should Clinton pick Warren as her running mate. On Monday, Warren will join Clinton for their first joint event at a rally in Cincinnati. The setting of the event — a Rust Belt battleground state where Trump has shown appeal — underscores how Clinton’s campaign believes they can use Warren to push her economic message.

Warren is expected to endorse most of Clinton’s proposals and cast Trump as putting forward plans that would benefit only the rich.

For Clinton, the visit offers an important opportunity to win back some of the liberal and younger voters she lost to Sanders in the primary. Though Sanders said on Friday that he’d vote for Clinton, he’s shied away from offering a full-throated endorsement or urging his supporters to back her.