Lipinski's campaign already took in more than $920,000 through the end of February, more than it has ever raised previously, according to the Federal Election Commission. Newman's campaign had a fundraising edge at the end of last month, taking in more than $1 million. Lipinski's committee crucially had about $1 million more in cash on hand than Newman's at that point.

The primary in Illinois marks the latest in the fights pitting Democrats who consider themselves moderates against candidates to their left who argue for change or fresh blood. Backers of Lipinski and Newman have spent accordingly in a district where the Democratic primary winner will almost certainly win the general election.

Big money has flowed into Tuesday's fight for centrist Democratic Rep. Daniel Lipinski's Illinois 3rd District seat, which he has held since 2005. Outside groups have put millions into the Democratic primary to prop up both Lipinski and his challenger, marketing executive Marie Newman, who has cast herself as a more progressive option.

A House district in Chicago's suburbs is the latest battleground for the Democratic Party's future.

Meanwhile, in a sign of the national interest in the race, outside organizations have piled more than $2.6 million into the primary, according to the Center for Responsive Politics. Some of those groups have filled the cash void for Newman. About $1.3 million of the spending has gone toward opposing Lipinski, largely from a newly spawned Super PAC called Citizens for a Better Illinois. Groups such as the pro-life Susan B. Anthony List have spent more than $900,000 to oppose Newman.

That cash flow underscores the national interest in the race as a proxy for the direction of the Democratic Party. Outside groups spent only a tiny fraction of this year's total in the district in the 2016, 2014, 2012 and 2010 elections combined.

Lipinski, as co-chair of the Blue Dog Coalition, has cast himself as a centrist who can cooperate with Republicans. He opposes abortion rights and voted against the Affordable Care Act when the House passed it in 2009.

The congressman has come under increasing electoral threat as Democratic voters have appeared to move further left. Newman has called him a "Trump Democrat" and attacked him for his abortion stance and health-care vote.

Lipinski has repeatedly highlighted his efforts to oppose the tax plan and health-care overhaul backed by President Donald Trump.

The race has left the national Democratic Party with tough choices. House Democrats' campaign arm, the Democratic Congressional Campaign Committee, has not outwardly endorsed Lipinski.

Meanwhile, independent Sen. Bernie Sanders of Vermont — a progressive leader — and Democratic Sen. Kirsten Gillibrand of New York have both backed Newman. Sanders and Gillibrand are often mentioned as potential Democratic candidates for president in 2020.

The contest between Lipinski and Newman will likely determine the eventual winner of the general election. The congressman and his father, William Lipinski, have held the seat since 1993. It is considered a safe Democratic district.

Polling in the race has been scarce. One survey conducted in February by Democratic polling firm Public Policy Polling found Lipinski had a 2-percentage-point advantage, within the survey's margin of error.

Arthur Jones is running unopposed on the Republican side of the race. He is a Holocaust denier whom the state's Republican Party has disavowed.