U.S. Rep. Betty McCollum, D-Minn., has joined the growing number of Democrats asking President Biden to withdraw his re-election bid. The congresswoman Friday endorsed Vice President Kamala Harris as the party's standard-bearer and suggested Minnesota Gov. Tim Walz as her running mate.

"Winning in November and defeating [former President Donald] Trump's dangerous, hate-filled agenda must be Democrats' sole focus," McCollum said in a statement. "To give Democrats a strong, viable path to winning the White House, I am calling upon President Biden to release his delegates and empower Vice President Harris to step forward to become the Democratic nominee for president."

McCollum's move against Biden comes as more Democratic lawmakers have called on the 81-year-old president to end his reelection bid, a turn of events largely stemming from what was widely viewed as a disastrous debate against former President Donald Trump at the end of June.

The Associated Press reported that Biden, who was also diagnosed with COVID-19 this week, was huddling with family and a few longtime aides at his beach house in Delaware on Friday as he resists the efforts to push him aside. Campaign chair Jen O'Malley Dillon said on MSNBC that the campaign knows there's been slippage in support for Biden but that they still see "multiple paths" to beating Trump.

"We have a lot of work to do to reassure the American people that, yes, he's old, but he can win," O'Malley Dillon said.

McCollum's public call came the day after Trump accepted the Republican presidential nomination during the party's national convention in Wisconsin, where he announced Sen. JD Vance of Ohio as his running mate. McCollum suggested that Walz, her former colleague in the Minnesota delegation, would be an effective rival for Vance.

"If she becomes our Democratic nominee, Vice President Harris will need a strong Midwestern running mate and I encourage her and the Democratic delegates to consider a successful leader who has been a teacher, soldier, football coach, former member of Congress, and a proven winner — Minnesota's Gov. Tim Walz," McCollum said.

McCollum, the senior member of Congress from Minnesota, has represented her St. Paul-area district for more than two decades. She sits on the powerful Appropriations Committee and has long been an ally of former Speaker Nancy Pelosi.

Distinguishing McCollum from some of the more vulnerable Democratic officeholders who have called on Biden to step down is that she has typically faced only nominal opposition in her strongly Democratic district.

Late Friday, Ohio Sen. Sherrod Brown, a Democrat who is in a tough race for reelection, called for Biden to step aside, the Associated Press reported. That brings the total number of Democratic members of Congress urging him to drop out to nearly three dozen.

McCollum is the second Minnesota Democrat to urge Biden to step down. Rep. Angie Craig, who represents a largely suburban swing district, made the same request days after Biden's debate against Trump. Rep. Dean Phillips, who earlier this year pursued an unsuccessful challenge to Biden for the presidential nomination built around a critique of Biden's fitness for office, has so far refrained from demanding he step down.

Rep. Ilhan Omar is the only Democrat in the state's House delegation to back Biden's bid for a second term. In a social media post Thursday, she said it was "shameful" that several Democratic leaders anonymously called for the president to step down.

"It's a lack of leadership, and it's making all Democrats look bad," Omar wrote on X. "Whatever this mess leads to will not undo the damage that has already been inflicted. May God help us all."

Sens. Amy Klobuchar and Tina Smith on Friday said Biden himself must decide whether to remain at the top of the party's ticket. Three of their fellow Senate Democrats have now called on Biden to step down, including New Mexico Sen. Martin Heinrich on Friday.

Walz echoed Klobuchar and Smith on Thursday, adding that Democratic voters cast their primary votes based on priorities that include reproductive rights, climate change and labor rights.

While Biden won Minnesota and all other states that held primaries, easily clinching the delegates needed to win the nomination ahead of the convention, Walz left the door open for another nominee.

"The decision of who that candidate will be really falls upon President Biden at this time," Walz said.

Star Tribune staff writer Chris Vondracek and the Associated Press contributed to this story.