(CNN)The strength of the challenge that Democratic Rep. Dan Lipinski is facing from progressive candidate Marie Newman in Tuesday's Illinois primary is the latest sign that Democratic voters have moved to the left on many issues, including abortion.
In the Democratic Party, there's little room for dissent on abortion anymore
Lipinski is a conservative, anti-abortion House member. Left-leaning outside groups and some prominent liberals, including Sens. Kirsten Gillibrand of New York and Bernie Sanders, a Vermont independent who caucuses with the Democrats, have endorsed Newman in the primary.
Lipinski has always represented a district that has been Democratic leaning. When he was first elected in 2004, Democratic presidential nominee John Kerry carried the district by 18 percentage points. This past presidential election, Hillary Clinton won it by a nearly identical 15 percentage points.
What has changed, though, is Democratic voters in general.
Lipinski has voted with President Donald Trump 35% of the time, which is more than you'd expect for a member of Congress representing a district as Democratic leaning as Lipinski's. The only three House Democrats who have voted with the President more and represented more Democratic leaning districts on the presidential level, Jim Costa of California, Henry Cuellar of Texas and Kyrsten Sinema of Arizona, have also been elected with less than 57% of the vote at least once. Lipinski has never won with less than 65% of the vote even in bad Democratic years, on the other hand.
Back in 2004, a Democrat from a safe district voting with the president as often as Lipinski might have been tolerated. Gallup found that moderate and conservative Democrats made up more than 60% of all Democrats in 2004. Liberals made up less than 35%.
Today, it's an entirely different ball game. Liberal Democrats, at 50%, are a greater force than the combined 48% of moderate and conservative Democrats.
We can see the change in the Democratic electorate on the specific issue of abortion, as well. According to a Quinnipiac University survey in December 2004, 67% of self-identified Democrats said abortion should be legal in all or most cases. Still, a significant minority, 27%, said it should be illegal most or all of the time.
Today, there are far more pro-abortion-rights voters in the Democratic electorate. In a Quinnipiac poll taken in December 2017, 82% of Democrats said abortion should be legal in all or most cases, a 15 percentage point increase from 2004. Meanwhile, only 14% said it should always or mostly be illegal, a drop of nearly half from 2004.
Further, the importance of the abortion issue has gone up among pro-abortion-rights Americans. A Gallup survey taken in 2004 found that just 10% of those who identified as "pro-choice" said they could vote only for a candidate who was "pro-choice." In a May 2016 poll, 17% of "pro-choice" Americans told Gallup they could vote only for a candidate who agreed with them on abortion -- a 7-point increase from 2004.
The big question for Democrats going forward is what will happen in future primaries. Although there aren't many members of Congress with Lipinski's record left to challenge, it's unclear how liberal one needs to be to avoid a challenge. That's especially the case with abortion, where being against abortion rights seems like a nonstarter in the Democratic Party. Even Doug Jones of Alabama and Conor Lamb of Pennsylvania, who were running in a deep red state and district, respectively, were for abortion rights.