Day stressed the importance of Lipinski's role as co-chair of the bipartisan Congressional Pro-Life Caucus.
"He's been a huge voice for the 21 million pro-life Democrats who see him as our leader," Day said. "He's the one we can count on. He's fighting there for us in Congress in trying to push the Democratic Party to be more understanding and supportive of our point of view."
"His voice there is hugely important in making sure the life issue continues to be bipartisan," she said, warning that the elimination of pro-life Democrats in Congress would harm the pro-life movement as a whole.
In the wake of claims Planned Parenthood violated laws against selling fetal tissue for profit, in 2015 Lipinski voted to defund the organization and transfer its funding to over 13,500 other health clinics--legislation which failed to pass the Senate. He co-sponsored a bill to add the criminal penalty of murder to the Born Alive Infants Protection Act.
In 2010, he was one of the few Democrats to vote against the Affordable Health Care Act, popularly known as Obamacare, and later backed legislation intended to address concerns about abortion funding and religious freedom violations enabled by the initial legislation and President Barack Obama's related executive order.
Lipinski attributed his primary challenge to an effort among some Democratic Party activists to create a "Tea Party of the Left."
"They want to have a party that is pure as they define that, and so I think that's a big reason for that (primary challenge) right now," he said, warning that efforts to narrow the party would harm its prospects for electoral success.
"Democrats need to be a 'big tent' party," he said.
"I think it's terrible for the party at this time when we've lost over 1,000 seats nationwide since 2010. At the 2014 election, Democrats in the House were at our lowest point since the Great Depression when Herbert Hoover was president. We've come up a little bit since then but we still have a big deficit to make up."
Day also thought campaigns against pro-life Democratic officeholders would harm the political prospects of Democrats, about 30 percent of whom self-describe as pro-life.
"I really believe the Democratic Party is listening to the wrong people right now. Abortion on demand, taxpayer funding, pushing pro-lifers out of the party is not going to be a strategy that's going to help us build," she said. "This doesn't feel like a good long-term strategy to be not backing a loyal Democrat."
(Story continues below)
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Lipinski said that pro-life Democrats who stand for election face a tough fight.
"The issue is more on the outside than inside the party," he said. "There are activist groups who have a lot of influence in Democratic primaries who are on the other side. They want to defeat pro-life Democrats in primaries. That makes it much more difficult."
"The party needs to step up and say 'we will support pro-life candidates who want to run for office,'" he added. "I think the pro-life groups need to step up and understand that there's a need to have pro-life people in both parties, and they need to step up for pro-life Democrats in primaries."
Day said abortion backers will find their own candidates to run against pro-lifers.
"The abortion lobby has a history of targeting pro-life Democrats," she said. In her view, the massive expenditures in a safe Democratic district "shows that the abortion lobby cares more bout their own issue than the health of the party."
Lipinski, a member of the Blue Dog Democrat coalition, has the endorsement of many labor unions, and both major Chicago newspapers. On March 1, he secured the endorsement of House Minority Leader Nancy Pelosi (D-Calif.).